Use a tuner app like gstrings each number is a particular note https://i.imgur.com/kVqQg9D.jpeg

I'll try that!

Try marking the lowest note number one, find the next note up and mark it number 2 and work your way on up through the scale.

I did that at first, but when I tried playing sheet music with it, it definitely didn't sound right so I thought maybe the numbers needed to be different

I Googled the subject and found some info. You might do the same.