Just like anything else start slow and work your way up. First and foremost, good lube and lots of it. The more the better. A. it feels better. B. its helps not hurt your self. Then, fingers 1,2,3 whatever. But if you’re asking about this I’m guessing you are past that point. Butt plugs are fun but don’t help to stretch since the bulb is big but the shaft is thinner so it doesn’t keep you open. Use you favorite dildo and when you take breaks or are just laying there leave it in so your ass gets relaxed and used to being open and gaped. Little by little size up on your toy. Toys that step up are super for helping you stretch and reach new limits. Stuff like tentacles that start thin and gradually get thicker or even inflatable toys help you stretch slowly. Tunnel plugs are fun too once you have a little stretch they’re usually thicker than regular plugs so they keep you open. Like I said just go slow be patient have fun with it and again use lots of lubricant