Lubrication, relaxation and start with a finger first. Once you have it in there and thats ok. Slide your dildo in, go very slowly. Your anus is a muscle and will accommodate so you just need to give it time. You can speed it up by using poppers, this not only relaxes your muscles in your throat and anus, but gives you a surge of feeling very horny and sexy. I use a dildo quite often and use poppers for the rush. Its one of my favourite ways to masturbate.
I use dildo’s I have three that are different sizes and I have 5 butt plugs that are different sizes and I use them a lot