Independently from the specifics of the VENI grants scheme, I’d like to try to say something on applying for grants more in general, based on what I found useful.
The starting point for all kinds of applications is that you do actually know your stuff and can produce some sort of “proof” of all this through your publication list, CV, letters of reference, etc. The funders need to feel confident in your abilities to successfully complete the proposed project.
The most obvious advice of all probably is to first read the guidelines and the supporting material for the application form, then look for other official documents that help define the parameters of the grant. What do they mean exactly when they talk about impact, innovation, valorisation, etc.? What structure, headings and keywords are they looking for? Make sure that your research project meets their terms. It certainly helps a lot if you can peruse previously successful applications, even better if you can get pointers from former successful applicants.
Don’t think you have to do this completely on your own: feedback from your mentors and your peers, possibly from outside your own specialism, can help you identify the tacit assumptions you’re making. Think about who is going to decide about your application: a general commission, or experts from your niche? If the former, getting feedback on your proposal from people outside your field at an early stage is invaluable. Don’t be afraid of criticism: better to hear from your mentors now that your proposal doesn’t cut it (and how to fix it), than getting rejected for the grant later because you were afraid of their disapproval. You don’t want to hear that it probably is “good enough”, you need to hear what’s wrong with it and keep improving it to get it among the top 10-20%.