"If you have two, I'd suggest one on each side of the neck, and near the bridge. If you can, use some double sided tape or small dots of hot glue and try a few spots to see if you like the sound better.
Glue the flat side to the box. …"
I suggest you keep that first build simple. A one string diddley or a classic fretless 3. Check out knotlenny's CBG 101 for a great starter lesson. Google it or find it at page top under resources/ how to play. Good wishes.