First off, discuss with your partner any reservations you may have about period sex. Talk things out to make sure everyone feels comfy.
Don’t forget to get rid of the tampon before making out if you are wearing one. (Yep, some people do forget and it gets stuck)
Use a towel over your sheets to prevent leaks. Alternatively, move to the bathroom.
Have wet wipes within reach to clean when you are done.
Make sure a condom is put on. You can still get pregnant or get an STI during the period.
Try different positions to find what feels best at this time. Spooning sex may be especially great now.