The PKGBUILD is basically a shell script, so you can see the commands it runs:
If you already have your modified source and your config it should be something like:
make prepare
unset LDFLAGS
make ${MAKEFLAGS} Image modules
# Generate device tree blobs with symbols to support applying device tree overlays in U-Boot
make ${MAKEFLAGS} DTC_FLAGS="-@" dtbs
This builds it.
It’s installed with:
make INSTALL_MOD_PATH=/usr" modules_install
make INSTALL_DTBS_PATH="/boot/dtbs" dtbs_install
cp arch/$KARCH/boot/Image "/boot"
But it’s all mentioned in the PKGBUILD I linked.