Hi. Some simple options:
1. Kenny's video shows a technique with the built in Reaper sampler:
https://www.youtube.com/watch?v=CkdksJGg214
2. This Reaperblog shows an excellent free drum sampler that is perhaps even easier to use:
https://www.youtube.com/watch?v=wdqTwMx0a5c
Then either play in the drums or place them manually on the grid.
3. Try MT Powerdrummer:
https://www.youtube.com/watch?v=Dd95wFRoQZM
This is great for rock styles and can be played as above but also comes with some grooves that you just drop into a track. If the grooves are too elaborate for your needs you can of course edit them in Reaper (just double click the item to get to the MIDI editor).
I'm not a great user of drums but these have worked for me in the past. Hope that helps.