As the others have suggested there are some key elements:
1. Go really slow. When you feel pain - stop and go back. You either need more lube or more time.
2. Don't mistake pressure for pain. The proper opening of the anus for penetration involves pressure - that is what you learn to enjoy.
3. There are two sphincter muscles - outer and inner. Make sure that both are well lubed and opened slowly.
4. A lot of not penetrative anal/perineum stimulation can help you relax. This will inevitably result in finger penetration - keep reapplying lube.
5. Once you can tolerate - even enjoy 1 finger, move on to a second finger. If your partner has large hands, coarse fingernails or is just too impatient; go from one finger to a smoothly tapered toy. My wife's favorite is the Tantus B-Bomb.
6. Once the the B-Bomb settles past both sphincters, just relax and enjoy the sensation and vibration. Have your husband keep up the foreplay by stimulating other areas. After a few minutes you can ask him to start gently moving the plug in and out. Not all the way, just so you can feel the pressure increase and decrease with the diameter of the toy.
7. Whenever the plug pops out - either on purpose or not, reapply lots of lube. I'm not that large, so whenever the plug can be easily moved in and out, my wife is ready for me to penetrate her. Especially early-on, once I feel her second sphincter yield to me - I stop to allow her to adjust to my presence inside of her.
8. I do not begin moving until she says its OK. Footnote - after a few months of regular anal sex - this all gets much easier. Thrusting should progress from slow & shallow to deeper and faster - on your direction. Help your partner by providing a ton of feedback. Tell him what's working and what's not. Don't assume pain will go away - when you experience pain - go back one step and find an enjoyable place to be.
9. Later on we found that using a lube shooter that gets the lube deeper inside which makes deeper penetration with a penis or dildo mush more comfortable.
10. Even after initial penetration it is likely that you will need additional lube. There is a trade of here. The lube will help your comfort level - but it requires an other penetration - so remember to relax for the second penetration and make sure your husband stops after each penetration before thrusting.
It has been a huge surprise to me how my wife's view of anal sex has evolved. She has gone from no-way to 'f-ck my ass now' in light speed. I hope you find your comfort zone and can begin to enjoy this wonderful sharing experience in the very near future.