Assuming we’re talking about your butt.
First, get really horny, really want it. Then play with smaller toys slipping them in and out, hold them in at their widest points for a bit too to stretch your anus out.
You just have to work up to it. It helps to have toys that steadily go up in girth, it’s really the only way. You can’t just go straight for the knot, at least I can’t.
You’ll get there though, I’ve taken knots and lifecast horsecocks with this method, even after a long time between sessions. I’ll go from super tight to stuffing myself silly.
Also lube, a shitton of lube, more lube than you think you need. I really like Bad Dragons cum lube, I find it works really well with big anal toys.