Hey!

I hope I can give you any help with my thoughts. (sorry for my bad english)

First I would relax, take a few deep breaths...

I would start fantasizing about a scenario with your husband that involves anal play with him and that YOU find arousing. For me (being a husband, too) the most important thing is that my partner feels good and also aroused with what she is doing.
I think fantasizing is the key in this situation. Is there any situation you can feel comfortable with?

I think about starting very slow. If you start that slow with anal play you will get so much nonverbal feedback from your partner. I just start with a little plot I would think about first time anal, maybe you find it inspiring. (contains detailed descripition )

If I were a woman I would make him lie to the bed on his back (and tie him). It would be best if he has his feet flat on the bed, his knees bent upward. Of course he would be nude and I would start teasing him, caressing his body with fingertips and maybe feathers or silk. Just for a short time to get in the mood. His body language will tell you, when it's time to change the strategy. Just relax, feel your breath, enjoy.
He will love being caressed by you and he will appreciate it.
Then you could start teasing his dick until he is aroused. Just the way YOU like him.
If it's the first time anal for him I think it would be best not to start with toys but with your hand. Rubber gloves make the hands smoother and of course are more hygienic. I would use lube on one hand with the rubber gloves and start on his spincter. He will certainly show you what feels good with his body language and his paraverbal sounds.
Don't enter too quickly, maybe just circle around the entrance. Relax, enjoy...,
I would continue this for a short time, combining with teasing his cock. He will give you tons of feedback when it's time to enter the next step.
Then you could start slightly pressing only one cup of one finger into. Relax. Watch. How does he react? Keep it in for a few seconds, maybe tease his cock. Then you can put your fingercup in again and start a very very very slow penetration, just with the cup of your finger.
Just watch his feedback. He will give you feedback with his breath: if it becomes deeper you are on the right way, if it becomes more flat, do something different, maybe focus on his dick again.

If you are really searching for his prostate you use one finger with palms up. slowly enter. Really slow. use enough of lube. You can find it on the upper side. If you found it, just rest on it, don't stimulate too quicky. Let his breath be your guide. The prostate must be slowly being awaked, Sometimes it's enough just to apply the slightest pressure on it like a butterfly that flies over a leaf.
Some men like prostate play very meditative, some like it quicker. He will give you so much feedback. As long as he has deep breaths you are on the right way.

I hope I could help you a little bit.