I've seen a lot of questions about how to get audio to fit to the grid on Reaper. It's an important step in the editing process, for sure, but here are two methods you can use to save yourself some time.
There's one more method that's a little more complicated (it's a good blend of these two), feel free to comment if you want to see that one, as well!
1:04 - Method 1: Slice and Slip
3:01 - Method 2: Dynamic Split and Quantize