We’ve tried a few times and he’s been most comfortable face down with his hips slightly raised. If he was facing me he felt more shy about it whereas facing away allowed him to get more into it :)
i start by putting him on his knees with his ass in the air and his tackle dangling free. I start with a lick of his manhood to get him going and then move down the shaft until I reach his ball sack, there I stop for a while licking him and taking each ball gently into my mouth. By now I have him highly aroused. I then start to slowly rim him whilst stroking his balls, finally I tease his erection until he unloads in my hand. This drives him mad.
I think the easiest and best method is to get your partner to lay flat on his back and start to work on his manhood with your mouth, run your tongue down the balls and gently pull his penis up so you get access to his ass. A few gentle licks to break him in and if he enjoys it you can always try multiple positions such as doggystyle so you can work on his member with your hands and you have full access to his behind.
Try him on his back, legs in air and near the edge of the bed so you can kneel. From that position you can start with BJ and then work your way down. Being doggy style etc can feel very exposed for a newbie.
The first time I was rimmed I was on side with my legs curled up a bit. She had been licking my balls and then surprised me by moving to pastures new. I found it very erotic.
Have tried other positions but find on my side the most relaxed for both of us.
Usually the wife rims me in the doggie position, then after rimming, I'm in the right position to be either fisted or pegged. Another she likes, is for her to lie on the bed with her head just on the edge of it and with me standing above her then lowering my arse on to her waiting tongue. Which ever position she choices, she Fantastic at rimming.