Create OC, find boards that seem like they would be interested in yours (similar moderation, similar topics) and form friendships with them, advertise through images that are related to both your community as well as the community you're advertising to, be autistic enough in the beginning to create multiple threads every day and even samefag them with content in the beginning if necessary, advertise on other websites that might know about 8ch but not know about your small board, bathroompost (write links to your board in public bathroom stalls), "e-bathroompost" (advertise in the comment sections of weird obscure places on the internet, but don't make yourself sound like a bot). There's nothing wrong with advertising your board on the most popular boards, or advertising your alternative on the board you're trying to split from, but do so in meta threads that already exist and try to monitor the thread so you're talking to people who are already interested in leaving. Some people may get pissed and try to raid you or falseflag your board to give newfags a negative impression. Don't be caught off guard.
It will be an arduous process and it will take months to pay off, unless your board gets its start through a large migration. Never forget that for every minute you spend shilling you should spend two minutes trying to make your board a better place. Never wait for other people to post something you want, you will need to actively post anything you want to see. If your board has a topic, never let your focus on the board prevent you from enjoying that topic. It's easy to become a metawhiner, but that isn't going to solve your problems.