Here's one recipe:
1. Two willing people
2. Lots of lubricant
3. Condoms to reduce contact with natural e-coli bacteria, STDs and risk of infection.
4. Knowing how to relax muscles to prevent undue discomfort, to include foreplay and stimulation during the act. And agreeing to stop if any discomfort crops up. Never force the issue.
5. A nice long shower afterwards.