Hello!

I do love anally fisting boys.

When you say plastic gloves, I hope you don't mean washing up gloves! Medical style latex gloves are what you want, and the ones which go to your wrist should be OK. If you plan on going deeper then I recommend farming insemination gloves or elbow length latex gloves. You do not want any skin to skin contact at all.

Tell him you have never anally fisted a man before. The best lessons I ever had about how to anally fist a man were from my clients who wanted it - they will be happy to guide you and lead you through it.

Have him on his back so you can communicate clearly, and use your left hand as this way it guides through the lower colon easier. Use lots and lots of lube and go slow. There are no nerve endings once you're deep inside so you are depending on his communication to make sure you don't damage him. You'll be able to feel your way through fairly easily though.

Let me know if I can assist any more

Also remind them they need to prepare    Ask them if they know what this entails and if so then what it is they do, to ensure they do so thoroughly otherwise your hands will be a-wading. 

Make he sure he douches beforehand 

In addition to that, you need to ask if he has been fisted before. If his answer is no, then you do not want to be the first.

Yes you need to ask him why he wants you to be the first to fist him when you've never done it before...