You have probably picked out the best method by default! Index finger on the button closest to the top, middle finger on the next one down, ring finger on the third. Little finger moves from button 4 to the little used button 5. Same on both sides. Right thumb for the heavy breathing button :-) Use the same on both rows FOR NOW! Once you start crossing rows it can get more interesting but stick with single rows in the early stages. With a 20 button there is not much scope to cross rows or play multi-row chords anyway!

Hope this helps

Dave the Gnome