The best thing is that you can just completely go for it with your designs, but it's worth researching a little on card layouts and best sellers to get an idea of what works well. I've had cards that have completely flunked that I've spend days illustrating, then ones I knock up in half an hour sell five thousand cards, so it's a lot of trial and error!
I'd say the biggest change over the past year is that the company has grown into a HUGELY successful business, so there is a higher quality expected, but also a LOT more competition, with thousands of artists and illustrators uploading cards every day, it's quite saturated and does feel much harder than when I started.
When you upload cards they get accepted and put on sale to appear in searches, set as pre-release if they're for a special occasion like Christmas or valentine's Day, but also set as profile only. Profile only means that people can only buy that design through your personal link or clicking on your design name. This has happened a lot more lately with a huge percentage of cards not being accepted. It's hard to learn from as you're not given any reasons why they weren't deemed suitable for sale, but they do five a list of general reasons such as typos, too niche, already similar on the site etc etc. I really wish it was possible to have notes with rejected designs as it may just be one small thing that needs tweaking but usually means just scrapping the whole design or uploading elsewhere. As the Thortful team is still quite small I can appreciate why this isn't possible, but hopefully on the future it will be as it would save a lot of time for us creators.
Overall, I still highly rate Thortful as a means of added income for artists and designers or really anyone who wants a crack at designing cards as quite often the best sellers are super simple! It's harder now with more competition, but still such a fun way of earning a passive income that can be a pretty hefty amount of your card is lucky enough to surf the algorithm wave and be a bit with buyers. You can see some of my cards at Wafflemama here and Duck and mustard here.