Unless you’re app is just going to be one or two pages, a basic plan of how the app is structured (which page leads to another page, the hierarchy of sections etc) will help a lot. If you’re at all familiar with building websites then you’ll probably have heard the term sitemap before (the ‘site’ in sitemap actually relates to a website but I don’t think I like ‘appmap’ even though it’s more accurate in this case!).
You should be able to sketch this out on a single piece of paper (unless your app is more complicated, then blow the budget and use several).You can then either scan this in or re-create it in a spreadsheet, drawing program or using the drawing tools in Word (an example of an early stage sitemap is shown on the following page).
This is also something you’ll want to share with your supplier, possibly at the tender stage when you’re still searching for a developer, so they can get a better idea of what you’re actually after.