When planning your SaaS, select a project management tool you are going to use…If you not thinking of using one I would highly recommend doing so…There are several platforms on the market…& some are free.
I use Jira for sprint planning by Atlassian for most of our project development & Confluence for requirements documentation.
My advice when starting is simply to build out a backlog of tasks like functions & features you would like to cover. You may not get to all of the tasks if you are building out an MVP, but you can always get to them in the next round of development. Be clear on what each function & feature is to do specifically, no need to be technical if you are not but recommended linking your UI design or prototype.