Repairing Small Holes (under 5mm)
We found a fantastic informational video from Professor Pincushion on YouTube about repairing tiny holes in woven fabric like cotton T-shirts, sweatshirts and hoodies. These two methods they show here is for holes under 5mm.
First things first, to make sure that the repair job (if you go for the needle and thread technique shown below) you are doing looks as invisible as possible, choose a thread that is the same colour as the surrounding fabric on the garment. If you can't find the exact colour, it is recommended to use a thread that is slightly darker than the garment, as it is likely to show up as much as a lighter colour.
We also recommend using a thread that is the same weight as the fabric you are repairing. For example, fixing a light material (like a silk shirt) should be done with a nylon or silk thread, whereas a heavier material like a cotton T-shirt will be better fixed with cotton thread.
And we also found a pretty nifty article on how to tie a knot in the end of your thread (before you start stitching). The one in this video is number 1 on this list: