All ribs need to have a center line drawn on them for where to place skin rivets.
48 ribs, two lines per rib and Yikes! A lot of lines!
There is an EAA/Sonex Aircraft video about using a Sharpie and holding the pen with your hand.
BUT
I spent a few bucks and got a great tool!
This is called an Edge Marker Block and is available from aircraftspruce.com
One guy to hold the rib, and one guy to mark the line. Fast and easy.
Remember not to use a black pen because blue, red or green will help you be sure you got the correct mark.
FYI: Here is the EAA/ Sonex Video to do this by hand.