I made this Zentangle Leaf with the pencils and used the Pentel Gel Roller for Fabric pen to outline in black. The pen works great on fabric; it rolls so smoothly with the lightest touch. I used all of one pen and a little from another, so a project like this can really "eat up" the ink. I ordered two boxes of 12 each, so I'm set for the near future. After doing the search below I could have bought them for less, but I got free shipping at the Amazon site.
Web search for Pentel Gel Roller for Fabric pen
Review of Pentel Gel Roller for Fabric - It was mentioned that this pen is at Hobby Lobby.
This is where I learned about these pens:
$1.68 each - Box of 12
$1.29 - $1.27 discounted prices for multiples
Zentangle Leaf 9 x 11 for an 8 x 10 frame