I'd say trial and error, mostly. If it helps, it took me maybe a couple of months to warm up and create a decent rentry, so don't put the pressure on yourself that you should be immediately at like "wow" levels of rentry-making(?), lol. During those months I've sort of dissected others' rentries to learn from them and how everything works... Maybe you could try the same? Unironically, take what Twilight said into account, sort of. A lot of rentries are just... handmade images- graphics- along with a lot of colors and text. Just experiment a lot, imo. Either way, you can't really go wrong. And you don't have to have a huge rentry with a boatload of content and images, either. Simpler rentries are nice, too!