Plain twist ties - plastic or paper (leave them attached together)
Markers - Permanent markers are best for plastic, almost anything works for the paper ones (I used sharpies and bic pastel permanent markers)
Ruler - if you prefer not to draw lines freehand
Draw stripes on the front first, then copy the lines in the same place on the back. My twist ties are slightly transparent so that makes it easy to follow the same pattern on the back. Diagonal and horizontal stripes work really well. It does not matter if the lines are not perfect as you won't even notice after the ties are separated.
I decorated this lot of twist ties with alcohol inks. (The Tim Holtz Ranger inks are the ones I have. You can purchase them at scrapbooking stores.) These are much quicker to make than the marker ties above and don't even matter if the front and back do not match.