Considering GM specifically WTA
As mentioned, as the GM, it's your job to push. While role playing in general can be a "dance" in that you need "two to tango" (player and GM) in play by post the GM is leading, and leading heavily. The game can't succeed without the players, and it will primarily be up to you to keep it going.
You'll have to drive the posting rate, keep up on recruitment (since attrition will happen), etc.
Set expectations that work for you, and make sure potential players know them. Do you want people to post daily? Tell them that. Are you strictly a monday-friday GM who is busy on weekends with real life stuff? Fine, but again, prospective players should have that information.
The better GMs on the site tend to have a set of guidelines they follow that are part of the information prospective players are given to read when checking out the game. Some even put codewords and such in the middle of the information that they ask you to include in the RTJ so they know whether you actually read it or not.
It can seem like a great idea to wait and get player input to build your setting and game, but don't. It more than likely isn't going to work the way it does around a table. Give people a setting they can build in and around. It needs to have plenty of things they can grab onto but you also don't want 100% of the setting filled. If someone has a better idea for something you can always throw away what you came up with and run with theirs, but don't expect a random pickup group to be able to do much world building.
As part of the expectations regarding a schedule touched on above, pacing is also going to be the most tricky thing you have to deal with. Time dilation is brutal in Play by Post so while you want to leave room for players to explore and have real agency, you don't want to make them have to do Every Little Thing. Be sure to give players enough information with each post that you can try and answer the majority of obvious questions. Avoid creating trivial obstacles if they're not important to the story you're trying to tell.
If players seem to be focusing on a red herring, tell them what's up and get them moving again (unless you feel like changing things so the red herring becomes the real thing).
If you want to include RNG drama (succeeding at notice checks and such), as part of that "expectation" talk above, let your prospective players know you'll be rolling certain dice for them as things go, and then supply them with the information they would know based on those rolls. Generally you want to avoid as much "call and response" when it comes to mechanics as possible. A 30 second "can I roll X to find out more", "Sure", *rolls*, "You see X, Y, and Z" exchange around a table may take several days. So keep in mind what rolls are important for players to make, how comfortable they are having you make said rolls on their behalf, and whether the success or failure of a roll is actually important from a story standpoint.
As for game specific advise.
One of the strengths of PbP is that you can split the party without half the table having to sit around and listen and/or be bored wile others play. So if one or more are going into the Umbra, cool. Post for them while also posting for the other folks. The only thing you'll really need to be aware of is getting into a situation where people are waiting. If a person or part of the pack is off having umbral adventures, make sure the rest have something to do. Also, unless there's drama or story potential to be had regarding stepping sideways, it's usually not worth making players roll for it as unintentionally splitting the party because someone failed and gets stuck on the other side for an hour is unnecessary.
Caerns. Use one out of the books if you're playing in one of the fleshed out areas, or use one of those as a model for whom you should have around. Assuming the players are going to be Cliath they're not likely to hold any positions of importance at the start. Your NPCs and their tribes should match the setting (urban vs rural vs wilderness) and, unless you're intentionally bringing in your players as a pack of strangers, should have hooks for players to latch onto if they so desire.
Totems. Not all totems are created equal. This isn't a PvP situation so mechanical balance should be easy enough to deal with. Just put some more weight on the side of whatever is opposing the player characters. Also, the stronger the totem (as in more benefits the players are getting), the more it's going to ask in return. Sure it might be easy for the pack to never eat wild boar if you're in the city, but maybe Boar comes calling and suddenly the pack also have to figure out to get the local zoo to release the pair it keeps in captivity and get some policy or agreement on the books to not acquire any more. You should also strictly enforce the concept behind the totem and what that would mean to wolves who worship it. If they're pledged to Easy Credit, they better be fully into the whole late stage capitalism thing.
Mechanically it's easier to cleave totem from individual backgrounds altogether. Once players decide on a totem (unless you're choosing to tell players exactly what sort of pack they're joining, including a pre-assigned totem) give them an initial investment that would represent everyone spending 1 point, for instance. Then if they want more, give them story methods to gain more favor.
It's a little more work than just having them spend points, but it prevents the horse trading of someone spending a bunch of points on totem while another only puts in 1, and then there's pressure for everyone to figure out how to put in X amount, etc.
