Override package with local build

I know how to use an overlay to build a different revision of a package, via Overlays - Official NixOS Wiki . However, the build process for the package takes a long time and a lot of storage space, and it seems to rebuild the package whenever I run sudo nixos-rebuild switch --upgrade. I would like to build the program with the override once, locally, then replace the overlay with my local version of the package, but I cannot find any documentation on how to do that. Please help! TIA