How do I put in a custom drum sample?
You can add any samples from the Import dialogue. Hit ctrl-i, or go to Session -> Import, and browse to your sample directory. Here’s the manual page: http://manual.ardour.org/adding-pre-existing-material/import-dialog/
I don’t think the manual mentions it, but I believe one can also drag and drop samples directly into tracks from the Import window.
does this work with custom drum sounds so you can make a beat with custom sounds?
I’m not entirely sure what you mean by custom drum sounds. With the Import feature you can add any sound file on your hard drive into the project (as long as it’s in a format Ardour supports—most samples you find online are probably wav files, which should work fine). Then in the editor, you can move and copy the sound clips as much as you want to create beats. An alternative is to use a sampler plugin, but that’s a whole other topic.