I always use Sakura Pigma Micron Pens for drawing mandalas. They come in various sizes and colours but most people start simply using black ink. The size I use most is the Micron 05 (0.45mm nib). For finer lines, I use a Micron 01 (0.25mm nib) and I use the Pigma Brush in the same range, also in black, to fill in larger areas.

These Sakura pens don’t bleed or smudge so are fantastic to use for drawings. They are also archival quality, and won’t fade so they are ideal for work you want to keep. And, being waterproof, you can go on to paint over them if you wish.