Firstly, look through the portfolio of these start-ups to understand the verticals they have worked with. Look at their preference in terms of the maturity level of a start-up - do they accept early stage, mid stage or late stage startups?
Ideally, they do not entertain start-ups who are only at the ideation space. There should a MVP or an alpha version of your product in place and an initial market research done. From my experience of working with NUMA, team matters a lot. It is preferred if the roles among the team members are well defined. The expertise should preferably be within the team.
Along with the prototype, work a little on your business plan. Of-course, no one expects you to have it all figured out but it is important that you have given a thought to all the elements that'd go into your business plan. Specially, a good competitor landscape analysis. You must know who your competition is. Also, work on the basic model of monetizing your idea.