First time anal tips.

My partner and I want to try anal. We've toyed around with that area before using this finger but that's it, and that's always a bit uncomfortable, even if we're properly lubricated. 
I'm not that bothered about it hurting, but I am absolutely terrified of doing it and pooping on him! 
What can I so to make sure that doesn't happen? 
Does anybody have any tips? 
Thank you.