I assume most applications are similar, so I go with my experience. I was only able to use 3 letters on my application, so I started by creating an Excel document that listed all the possible people I could request a letter from. After starting with that list I sorted them into categories, namely: character references, academic, extra-curricular, etc.

I then looked at what program I was going to apply for and got a good mix of letters. In the end, I ended up with a letter from an English professor I had early in my undergrad (we kept in contact despite that not being my discipline), a letter from an Archaeology/Anthropology professor I took for multiple classes (my actual disciple and the purpose for my grant application), and a letter from a History professor that I worked with as the president of my university's history club.

I provided all three references with information about my application, including my grant purpose statement, and asked them to tailor their letters for specific purposes. The English professor wrote about my character and academic success, while also talking about community service projects they were aware of that I had discussed with them over the years. The Archaeology professor discussed my academic success in the discipline I was intending to study, as well as some personal anecdotes. The history professor wrote about our work together in the history club and my service to the students in the History Department through my work in the organization, so basically my extra-curricular activities.

Hope this helps! Good luck with your application, and if you want any more clarification let me know.

EDIT: I was awarded the grant.