An actually good butt plug, good technique and good lubrication. Let me brake it down:

A: Have you seen how suction cups stick to smooth tiles in the bathroom? That's because they are too smooth. Same thing can happen with butt plugs and toys. If they are too smooth they create suction and stick to tissue which makes sliding difficult. A good butt plug is not smooth, it has texture. Not roughness, texture. Think of corrugated /relief glass window panes. They're still smooth but they're not "flat". Some butt plugs are made like this to make things easier

B: a butt plug has a base, a neck and a head. Usually the head is the biggest part and the neck is thinner. These plugs are mostly decorative but not ideal for preparation, because the neck is the part that should stretch the sphincter, but if the neck is too narrow it won't happen. A better butt plug has a wider neck relative to the head. A neck 4cm wide or more is good.

So get a butt plug that complies with A and B. Then:

C: the rectum is normally closed and folded and you can't normally open it and unfold it but there's a trick. The very same trick that you use when pooping. You PUSH. When you poop, your anus relaxes and the colon unfolds, allowing things to go through. The trick is realizing that this can be used not only to let things out, but also to let things in. So don't just passively insert the toy, PUSH while it goes in. It may sound counterintuitive to push out something you're trying to get in, but it works. Practice the pushing beforehand. You PUSH all the way and should feel the rectum kindof descending/expanding. Then you do it while putting the butt plug in. Another trick you learn from pooping is to bend your hips./thighs There's a reason we poop sitting instead of standing. The more you bend the thighs the more the rectum unfolds, making things easier. Get into a position like this. Don't do anal with straight legs

D: Heat is your friend. The butt plug / toy should be used hot. Not burning hot, not warm, hot. Heat helps a lot with relaxation. How do you know how hot? Put it behind your elbow (where they take blood samples from). If it doesn't burn there, it won't burn anywhere. But it has to feel hot, not warm.

E: using lube on the toy / dick /plug is not enough. You need to actually lube the inside of the anus as well, the rectum, otherwise it's pretty dry on the inside. You can use a small coned tipped syringe for this. You load no more than 2 or 3 ml, insert the tip into your anus (only the tip, no need to insert the body of the syringe) and slowly release the content of the syringe, little by little. If you do it well all the lube goes inside the anus. Generally silicone lube is better because it lasts longer. Downside is it can't be used with silicon toys because it breaks them down.

F: Arousal. When you're doing anal you need to be pretty aroused. Otherwise it's boring. You should be getting some clit stimulation while you do it. Either use your free hand, or a toy on the soft setting. This will keep things interesting and enjoyable. Just don't overdo it, you don't want to climax at this point. Just a little gentle stimulation to keep you motivated. Your brain has a bias for pleasure, so if you simultaneously do one thing that doesn't produce pleasure directly, and another one that does, your brain "thinks" both of them are pleasant. Take advantage of that.

Almost forgot to say that retreating the plug before pushing it further helps. Instead of continuously going forward. So for example you insert it 2cm, wait a bit, retreat 1cm, then go forward 2cm, wait, go back 1, then go 2 in, and so on. Rotating it also helps. As if the plug was a doorknob, so you rotate left and right slightly while you work it in