here are a few examples:

cat pkgbuild_autoconf2.13

pkgname=autoconf2.13
pkgver=1
pkgrev=1
pkgdesc="description here..."
url="url here"
groups=add
depends=()
makedepend=(js)
packager=""
tfext=


build() {
patch -Np1 -i $SRC/autoconf-2.13-consolidated_fixes-1.patch
mv -v autoconf.texi autoconf213.texi
rm -v autoconf.info

./configure --prefix=/usr --program-suffix=2.13
make
make check
make DESTDIR=$pkgdir install

install -v -m644 autoconf213.info $pkgdir/usr/share/info
install-info --info-dir=$pkgdir/usr/share/info autoconf213.info
}

cat pkgbuild_libpeas

pkgname=libpeas
pkgver=1.30.0
pkgrev=1
pkgdesc="moteur de greffons basé sur gobject etc..."
url="https://download.gnome.org/sources/libpeas/"
groups=add
depends=(gobject-introspection gtk3)
makedepend=(gedit)
packager=you
tfext=tar.gz


build() {

mkdir build
cd   build
meson --prefix=/usr --buildtype=release ..
ninja
ninja test
DESTDIR=$pkgdir ninja install

}

cat pkgbuild

pkgname=pm-utils
pkgver=1.4.1
pkgrev=
pkgdesc="description here"
url="https://pm-utils.freedesktop.org/releases"
groups=add
depends=(xmlto)
makedepend=()
packager=
tfext=

build() {
./configure --prefix=/usr    \
     --sysconfdir=/etc  \
     --docdir=/usr/share/doc/pm-utils-1.4.1
make
make DESTDIR=$pkgdir install

install -v -m644 man/*.1 $pkgdir/usr/share/man/man1
install -v -m644 man/*.8 $pkgdir/usr/share/man/man8

ln -sfv pm-action.8 /usr/share/man/man8/pm-suspend.8
ln -sfv pm-action.8 /usr/share/man/man8/pm-hibernate.8
ln -sfv pm-action.8 /usr/share/man/man8/pm-suspend-hybrid.8
 }

cat pkgbuild_kmod

pkgname=kmod
pkgver=29
pkgrev=1
pkgdesc="libraries and utilities for loading kernel modules"
url="https://www.kernel.org/pub/linux/utils/kernel/kmod/"
groups=base
depends=()
makedepend=()
packager=
tfext=tar.xz

build() {
./configure  --prefix=/usr   \
      --sysconfdir=/etc  \
      --with-openssl   \
      --with-xz      \
      --with-zstd     \
      --with-zlib

make
make DESTDIR=$pkgdir install

for target in depmod insmod modinfo modprobe rmmod; do
ln -sfv ../bin/kmod $pkgdir/usr/sbin/$target
done

ln -sfv kmod $pkgdir/usr/bin/lsmod
}

cat pkgbuild_pygments

pkgname=pygments
pkgver=2.12.0
pkgrev=
pkgdesc="general syntax highlighter written in python"
url=""
groups=add
depends=()
makedepend=(gtk-doc)
packager=
tfext=

build() {
pip3 wheel -w dist --no-build-isolation --no-deps $PWD
pip3 install --no-index --find-links dist --no-cache-dir Pygments --root=$pkgdir
}

cat pkgbuild_bzip2

pkgname=bzip2
pkgver=1.0.8
pkgrev=1
pkgdesc='programs for compressing and decompressing files'
url='https://www.sourceware.org/pub/bzip2'
groups=base
depends=()
makedepend=()
packager=
tfext=tar.gz

build() {
patch -Np1 -i $SRC/bzip2-1.0.8-install_docs-1.patch
sed -i 's@\(ln -s -f \)$(PREFIX)/bin/@\1@' Makefile
sed -i "s@(PREFIX)/man@(PREFIX)/share/man@g" Makefile

make -f Makefile-libbz2_so
make clean
make
make PREFIX=$pkgdir/usr install

cp -av libbz2.so.* $pkgdir/usr/lib
ln -sv libbz2.so.1.0.8 $pkgdir/usr/lib/libbz2.so
cp -v bzip2-shared $pkgdir/usr/bin/bzip2
for i in $pkgdir/usr/bin/{bzcat,bunzip2}; do
ln -sfv bzip2 $i
done
rm -fv $pkgdir/usr/lib/libbz2.a
}

cat pkgbuild_glibc

pkgname=glibc
pkgver=2.37
pkgrev=1
pkgdesc="main C library"
url="https://www.gnu.org/software/libc/"
groups=base
depends=(libidn2)
makedepend=()
packager=
tfext=

build() {
patch -Np1 -i $SRC/glibc-2.37-fhs-1.patch
mkdir -v build
cd    build
echo "rootsbindir=/usr/sbin" > configparms
../configure --prefix=/usr             \
      --disable-werror           \
      --enable-kernel=3.2         \
      --enable-stack-protector=strong     \
      --with-headers=/usr/include       \
      libc_cv_slibdir=/usr/lib

make
make check
touch /etc/ld.so.conf
sed '/test-installation/s@$(PERL)@echo not running@' -i ../Makefile
make DESTDIR=$pkgdir install
sed '/RTLDLIST=/s@/usr@@g' -i $pkgdir/usr/bin/ldd
mkdir -pv $pkgdir/etc/ld.so.conf.d
mkdir -pv $pkgdir/var/cache/nscd
mkdir -pv $pkgdir/usr/lib/locale
cp -v ../nscd/nscd.conf $pkgdir/etc/nscd.conf
install -v -Dm644 ../nscd/nscd.tmpfiles $pkgdir/usr/lib/tmpfiles.d/nscd.conf
install -v -Dm644 ../nscd/nscd.service $pkgdir/usr/lib/systemd/system/nscd.service

localedef -i POSIX --prefix=$pkgdir -f UTF-8 C.UTF-8 2> /dev/null || true
localedef -i ja_JP --prefix=$pkgdir -f SHIFT_JIS ja_JP.SIJS 2> /dev/null || true
localedef -i cs_CZ --prefix=$pkgdir -f UTF-8 cs_CZ.UTF-8
localedef -i de_DE --prefix=$pkgdir -f ISO-8859-1 de_DE
localedef -i de_DE@euro --prefix=$pkgdir -f ISO-8859-15 de_DE@euro
localedef -i de_DE --prefix=$pkgdir -f UTF-8 de_DE.UTF-8
localedef -i el_GR --prefix=$pkgdir -f ISO-8859-7 el_GR
localedef -i en_GB --prefix=$pkgdir -f UTF-8 en_GB.UTF-8
localedef -i en_US --prefix=$pkgdir -f ISO-8859-1 en_US
localedef -i en_US --prefix=$pkgdir -f UTF-8 en_US.UTF-8
localedef -i es_MX --prefix=$pkgdir -f ISO-8859-1 es_MX
localedef -i fa_IR --prefix=$pkgdir -f UTF-8 fa_IR
localedef -i fr_FR --prefix=$pkgdir -f ISO-8859-1 fr_FR
localedef -i fr_FR@euro --prefix=$pkgdir -f ISO-8859-15 fr_FR@euro
localedef -i fr_FR --prefix=$pkgdir -f UTF-8 fr_FR.UTF-8

cat > $pkgdir/etc/nsswitch.conf << "EOF"
# Begin /etc/nsswitch.conf

passwd: files
group: files
shadow: files

hosts: files dns
networks: files

protocols: files
services: files
ethers: files
rpc: files

# End /etc/nsswitch.conf
EOF


tar -xf $SRC/tzdata2021e.tar.gz
ZONEINFO=$pkgdir/usr/share/zoneinfo
mkdir -pv $ZONEINFO/{posix,right}
for tz in etcetera southamerica northamerica europe africa antarctica asia australasia backward; do
zic -L /dev/null -d $ZONEINFO ${tz}
zic -L /dev/null -d $ZONEINFO/posix ${tz}
zic -L leapseconds -d $ZONEINFO/right ${tz} 2>/dev/null
done

cp -v zone.tab zone1970.tab iso3166.tab $ZONEINFO
zic -d $ZONEINFO -p America/New_York
unset ZONEINFO


cat > $pkgdir/etc/ld.so.conf << "EOF"
# Begin /etc/ld.so.conf
/usr/local/lib
/opt/lib

EOF

cat >> $pkgdir/etc/ld.so.conf << "EOF"
# Add an include directory
include /etc/ld.so.conf.d/*.conf

EOF

}

cat pkgbuild_bluez

pkgname=bluez
pkgver=5.63
pkgrev=1
pkgdesc=""
url="https://www.kernel.org/pub/linux/bluetooth/"
groups=
depends=(dbus glib libical)
makedepend=()
packager=""
tfext=tar.xz

build() {
./configure --prefix=/usr      \
     --sysconfdir=/etc    \
     --localstatedir=/var   \
     --disable-manpages  \
     --enable-library
make
make DESTDIR=$pkgdir install

ln -svf ../libexec/bluetooth/bluetoothd /usr/sbin
install -v -dm755 $pkgdir/etc/bluetooth
install -v -m644 src/main.conf $pkgdir/etc/bluetooth/main.conf

cat > $pkgdir/etc/bluetooth/rfcomm.conf << EOF
# start rfcomm.conf
# ... subsystem in the Linux kernel.
# Use one line per command
# See the rfcomm man page for options


# End
EOF

}

cat pkgbuild_elfutils

pkgname=libelf
pkgver=0.187
pkgrev=1
pkgdesc="a library for handling ELF"
url="https://sourceware.org/elfutils/"
groups=base
depends=()
makedepend=()
packager=
tfext=tar.bz2

build() {
./configure  --prefix=/usr      \
       --disable-debuginfod   \
       --enable-libdebuginfod=dummy

make
make DESTDIR=$pkgdir -C libelf install

install -vm644 config/libelf.pc $pkgdir/usr/lib/pkgconfig
rm $pkgdir/usr/lib/libelf.a
}

cat pkgbuild_p7zip

pkgname=p7zip
pkgver=17.04
pkgrev=1
pkgdesc="command-line file archiver with high compression ratio"
url="https://downloads.sourceforge.net/p7zip/"
groups=base
depends=()
makedepend=()
packager=
tfext=

build() {
sed '/^gzip/d' -i install.sh
sed -i '160a if(_buffer == nullptr || _size == _pos) return E_FAIL;' CPP/7zip/Common/StreamObjects.cpp
make all3
make DEST_DIR=$pkgdir       \
   DEST_HOME=/usr       \
   DEST_MAN=/usr/share/man   \
   DEST_SHARE_DOC=/usr/share/doc/p7zip-$pkgver install
}

cat pkgbuild_linux

pkgname=linux
pkgver=6.4.1
pkgrev=1
pkgdesc="the Linux kernel"
url="https://www.kernel.org/"
groups=base
depends=()
makedepend=()
packager=
tfext=tar.xz

build() {
make mrproper
make menuconfig
make
make INSTALL_MOD_PATH=$pkgdir modules_install

mkdir -pv           $pkgdir/boot
cp -v arch/x86/boot/bzImage $pkgdir/boot/vmlinuz-$pkgver
cp -v System.map      $pkgdir/boot/System.map-$pkgver
cp -v .config         $pkgdir/boot/config-$pkgver
install -v -m755 -d      $pkgdir/etc/modprobe.d

cat > $pkgdir/etc/modprobe.d/usb.conf << EOF
install ohci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i ohci_hcd ; true
install uhci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i uhci_hcd ; true
EOF
}

cat pkgbuild_gobject-introspection

pkgname=gobject-introspection
pkgver=1.74.0
pkgrev=1
pkgdesc=""
url="https://download.gnome.org/sources/gobject-introspection"
groups=add
depends=(glib cairo gjs gtk-doc mako markdown)
makedepend=(glib)
packager=
tfext=

build() {
mkdir build
cd   build

meson --prefix=/usr --buildtype=release ..
ninja
ninja test
DESTDIR=$pkgdir ninja install
}

these examples are based on the 'BLFS PROJECT' if you are building a LFS/BLFS system I strongly encourage you to follow the official instructions of the project
you simply need to fill the instructions in 'pkgbuild' files