Choose a strap-on in a comfortable size to begin with. A shorter shaft length and pony length and smaller circumference will be more comfortable for a beginner than a huge strap-on. If you end up with a large double strap-on, insert it slowly bit by bit as you gauge how comfortable the increasing length is. Stop if it begins to feel uncomfortable. This applies both for solo play and when with a partner. As you get used to the strap-on, you'll be able to take in more length and even switch to a bigger dildo. Additionally, choose a flexible model so you can adjust the angles accordingly for greater comfort and stimulation. Some strap-ons are firm, but they are adjustable, so check that.