For the WACOM Volito pen, Spaghetti Nr. 5 is the right size (depending on your pen, choose the right diameter of Spaghetti):
Remove the original nib from the pen:
Cut the right length of the Spaghetti (1: original nib 2: Spaghetti-nib):
Push the Spaghetti-nib into the pen:
ALL DONE! Just sand the tip of the nib to your liking: