How to Install a Pull Request - KSP-RO/RP-1 GitHub Wiki
So, you want to download and install your favorite PR (Pull Request) because it's got some handy new features you want to try, but you can't bother to wait until the mod releases to try it out as intended. Be warned, not all pull requests are of good quality, and some might just break your game. (I should know)
There's 2 types of Pull Requests that we need to consider first.
For the first type, let's take a look at the Early Lunar Habitation program PR for RP-1, currently still in development as of the writing of this page.
Open up the "Files Changed" tab to see what this PR changes.
Now take a look at the left side of your screen to see what folders the changes take place in.
If all of the changes are contained within GameData (or, for RP-1, in Source/Tech Tree)*, then congratulations, you've got a GameData-only PR. All you need to do for this type of PR is open the repository that the PR is stored under.
*More specifically, if none of the changes include a change to a file ending with .cs, .csproj, and/or .sln.
Now, you can follow the standard process of downloading a mod from the source code, being careful not to delete any .dll files present in your original install. If you're downloading multiple PRs and they edit the same file, try to combine them in the least destructive way possible, keeping attention to how the changes were structured originally.
For the second type, let's take a look at this PR for RP-1. If you open up the "Files Changed" tab, you'll notice that there are files changed that are not in GameData or Source/Tech Tree.
This means that simply copying the repository's source code won't work if the source code doesn't contain an updated .dll file. (it most likely won't for a PR)
You have three options from here.
If you see a release page in the PR's repository, then they've likely built the .dll for you. How nice!
Open the release page, follow its instructions, and download the mod through there.
If not, go back to the conversations tab of the PR.
If you see a message that contains a .zip file, then great! That will work perfectly fine as well, just download that normally.
If still not, then you're mostly out of luck at this point. You could try either building the source code yourself to generate the .dll files, or asking the PR author (nicely) to generate them for you.