alright I'm going to assume you got all of the other binaries except for switch-zstd so lets start there:
go here:
https://github.com/devkitPro/pacman-packages/blob/4f8345f3d5c48e9bd97a269e015ab8db876e3ac9/switch/zstd/PKGBUILD
copy paste everything there to a blank file, save it as PKGBUILD...