Create a new instance using the version that you wish to use and make sure all resources needed are downloaded. Edit that instance and add the mod you want to install as a jarmod using the "Add to Minecraft.jar" feature.
Also install Manifold as a jarmod for inf-20100630 BigInteger by adding it the same way.
Navigate to your .minecraft folder and open versions. Make sure you have already
ran the version that the mod you want to install is based on. Make a copy of that
folder in your versions directory and rename it and the files inside to something
like version-mod where version is the Minecraft version base you are using and
mod is the mod you are installing (ie: 64bit, BigInt, etc). Edit the json and delete
everything between "client" and (but not including) "server". Then find where it
says "id": version and change the version to what you named the files previously.
Save everything and reopen your launcher. Please note, 1.3.2-BigInteger will likely
not work on PojavLauncher without patching.