There’s now a dedicated set of steps for pick-up bars independently of time signatures. If you’ve already got music in the project, you might also need to insert/remove beats to move the music into the pick-up bar.
There have been some examples on the forum and in the Facebook group in the past helping people fix-up a project when retro-actively putting a pick-up bar at the start; if you can’t find these or they’re not helpful, feel free to share a more specific example of where your project is at currently and what you want to achieve.
E.g. this one: Convert bar into pick-up bar