From Tie-dye Wiki
The diamond fold is an opposite sided accordion fold. I think it works best with the shirt inside out, fold in half longwise with the front to the inside. However with thick knits, maybe front to the outside.
I learned this fold from an old video tape by the two Dougs from Montana.
1. Establish the angle of the diamond and draw the lines to fold on, 45 degrees at the angle lines:
2. Start accordion folding from each end of the shirt - you have to push the shirt toward the center. When the beginning folds reach the center point, you have to alternate from top to bottom fold until the whole shirt is completely folded.
3.Tie - I am using Tom and Martine's technique with sinew, tying each section separately. The two Dougs use rubberbands and do it four handed. The first tied section at the center of the shirt should be small and tied tightly.
4. I apply the dye to the bands - On the following shirt, the dye was the same on both sides and I left out some sleeve from the tie to do a bleed.
Short sleeve shirts are easier to do than long sleeve.
I have done some since and applied different colors to the bands on each side. k. taltre