First steps to strapon
Try asking her to use a regular dildo in your ass first instead of a strapon. This will give you a chance to see if you like having another person be in control of the situation and it will give her a chance to penetrate your ass in a more passive manner. (Versus strapping on a dildo and fucking your ass with it, which really isn't passive at all on her part! lol)
If using a regular dildo works out okay, then ask her to progress to a strapon. Go to some of the sites like xandria or adam and eve and shop for a strapon together. For one thing, it is a lot of fun to check out the variety of strapons out there. Plus, it will give you both a chance to make a decision you are both fairly comfortable with. There are strapons out there that allow the woman to insert one end into her vagina when she straps it on. (The Feeldoe is an advanced type of strapon that doesn't need straps. The woman inserts one end inside her vagina and it is almost like she has a real penis.)
Let her know you like to have your ass stimulated. Let her wear a glove and use her fingers inside your ass. (Some women prefer to wear a glove, some go without.) Remember, sometimes things get dirty back there no matter how well you prepare, so the best way to handle it is to BE PREPARED. If you are doing anal play somewhere that might be a difficult cleanup, put down old towels, an old blanket, etc. Have some old hand towels ready, some dry, some wet and be ready to throw them away if she doesn't want to put them in the washer afterwards... Or, have some wet wipes handy and a small trashcan close by, ready to go. (Have a liner in the trashcan.) A roll of paper towels is handy to have close by, too.
Use lots of lube, such as KY Jelly or Astro Glide, or whatever your favorite lube is. Just remember, some lubes react badly with some dildo material. Plus, if a lube is heavily scented, it might cause a reaction in your anal area. I prefer KY Jelly because it is water based and cleans up easily. (Plus, it is unscented!)