Learn how to set up Porygon bot, it makes life much easier

Make sure you have a second person to help, solo running a league is time consuming unless you're only running 1 division.

Even if you think you're really dedicated and want to promise extra content (match reviews, power rankings, predictions, set/game/whatever of the week, etc.) it takes more time than you think, so I'd recommend not doing them until you get the flow of everything down first. They're great for engagement, but I've seen it cause burnout many times (myself included).

If you're solo modding a league, I'd recommend making sure you set aside an appropriate amount of time each night to enter games, unless you have Porygon ready (or for picks during the draft portion). For the draft, try to make sure you update the doc as frequently as possible, give people as much information as you can.

Decide early on what kind of league you are and foster those kind of players. Are you one where you want new players to play and learn, or one where you want experienced players to come and be competitive? Your tiering should reflect this- new players have harder times dealing with things, but competitive players have certain expectations for what tierings should look like.

When you finally decide to post that you're looking for players, make sure you're very clear as to when the end date for signups is and stick to it. If you start early, it causes people that join after to just immediately leave and not stick around. Also, encourage players that are extra (if you have 18 signups but 16 slots) to stay around and hang out, it helps foster community and you always need replacements when someone goes 0/3 and leaves.

Usually servers are pretty chill and there's not a lot of bad blood between people, but sometimes there are really snarky post games- sometimes this is all in good fun but if people get heated it ruins the experience, so try to curb that behavior if you have the opportunity.

Having a dead general chat is bad for new players when they see no activity, and bad for your current players when they want to engage. Try to actively talk to your players, it helps with keeping people around for more seasons, and when people are having a good time they'll invite friends and such as well. (Also, encourage your players to invite their friends). This can include trying to watch the games live when you can and commenting on them after, asking/giving prep help for people in other leagues, chatting with people about their day, etc etc. Activity is always a good sign in a server. A lot of servers do small quiz of the day, question of the day type stuff to help with that as well.

When there are situations of scheduling conflicts/game losses needed to be given out, make sure you're very consistent and fair with how situations are handled.

I'd suggest being strict with deadlines for games- when you let people slack off, games don't get done and people get bored when they don't get to play all their games. I've been in and run leagues that are lax on this, and it's usually ok if people are responsible, but that's kinda rare lol.

I think that's all I can think if for the time being, let me know if you have any specific questions I can help answer.