Easiest position? Lay on your side, and aloow him to enter you,either him laying on his side, or with him perpendicular to your body. Your body is more relaxed this way. Nico swears by it, and I have to say I agree with him. I was exclusively a top,but bottomed for the first time a little over three years ago. I was having a rough time of it,until Nico sugested this.
You would need to ask him for the mechanics of it all,as I don`t remember the mechanics of it all,but it works!
Alternately,seeing as how you vcan already take dildos, you can lay on your stomach, and have him slowly enter you. Lots of lube, and easing in and backing out help. I would recomend foreplay beforehand,to get you relaxed. If he is into it,have him rim you.
I will risk the wrath of Nico by advocating fingers to help stretch you out some. Make sure his hands are not too terribly rough,nails trimmed very short and filed. For extra smoothness, you can have him use a glove. If you feel pain or discomfort,have him stop. You may be one of the oddballs who don`t like fingerplay*
*Nico I adore you,but that is beyond odd to me.
Clean out before any play with WATER only. Use a bulb or gravity feed anal douche. Warm water only. Give your self a few hours before play to clean out and let the residual water reabsorb (there will be some)
Other than that, play safe, relax,speak up,and have fun! Let him know if you are experiencing any sort of difficulty,or if you need to pause or stop. Do not be discouraged if you cannot take all of him or go for very long before calling a halt to activities. It took me 3 or 4 times before I could bottom comfortably.
Hopefully Nico will chime in, but I know he is trying to limit his time here.