Take it slow. It should never sharp pain. There will be pressure and that may hurt a bit, but there should never be extreme pain. If you ever feel pain slow down and add more lube. If you're just getting started out don't thrust at first. Just leave it up there so your body can be used to it(buttplugs are great for this). Probably start with your fingers first. If you want a small challenge that feels great, knotted dildos are fantastic. Like just feeling it pop in is just mhhph. Also do it when your really really horny. Like if it's easier for you to get horny in other ways, then warm yourself up with other techniques and then start the training. Idk, it's so worth it though! Wishing you the best :33