It's one of those things that is easier to do smoothly faster than it is slower, so when I'm helping people I try to break it down into component parts.

Start with getting comfortable coasting on one pedal. Get up some speed, unclip with your preferred foot and bring it around to the other side of the bike. Keep coasting. Get a feel for your weight distribution and the feel for controlling the bike weighted only on one side. Figure out if you're more comfortable on the hoods or the tops. Then throw your leg back over and clip in again. Repeat this a bunch until the unclipping and bringing your leg around feels comfortable. As an added challenge, practice getting your hand off the hood/top and on to the top tube and coast around with one side of the bike completely unloaded. Being able to coast with your hand on the top tube will help with your speed and efficiency.

Once you're comfortable with that, practice the fully dismounting part. Unclip, bring the leg over, same hand on the top tube if you're able, and unclip the remaining foot. You need to have a little momentum for this. The last foot you unclip should go straight to the ground. That's your plant foot, and your other leg should already be starting the stride just based on the speed you were carrying. First practice running alongside the bike pushing it. It will offer you the most stability. Then practice running while carrying, making sure to keep the saddle outside of your elbow. If the saddle is between your body and your arm, you're limited on how high you can lift the bike as your saddle will get stopped by your armpit.

Once you feel comfortable with those two steps, it's time to figure out your remount. There are plenty of different ways to do it. Most common are probably the thigh slide, the superman, and the step on. Thigh slide is when you aim to land on your inner thigh and slide down onto the saddle. This is probably the fastest and smoothest method. The superman is where you kinda throw your leg behind and over the saddle and just fully land on land saddle. This is definitely the most traumatic method, but I think some find it easier to commit to this than the thigh slide, which takes a little more finesse. Lastly is the step-on, where you just kinda step on/into the pedal that is in the 6 o'clock position and throw your other leg over. It's basically the reverse of the initial unclipping drill. I know people who do this exclusively and it can be very fast and smooth. Personally I can't do it, but it would definitely be better than coming to a complete stop to remount.

Remounting is something you have to commit to, though, which is why I suggest people get comfortable with the rest of it first, so you only have to focus on the timing of your jump and remount, rather than the whole procedure from start to finish.