First of all, you need to source your jumpers. Here, you must make sure that all the jumpers that you use for making felt should be 100 per cent wool.
If, for any case, you do not own wool jumpers, then you can easily find them in charity shops. You can visit a nearby charity shop and look for jumpers with your desired colours. Then you can check the labels attached to the jumpers to make sure if they are made up of 100 per cent wool.
After you are done with acquiring jumpers, you will have to wash them to brighten their colours, as well as to get rid of the dirt or any other stain on them. For that purpose, put them in the washing machine and add washing detergent. Wash on quite a high temperature.
You can set the temperature higher than the maximum recommended temperature on the garment's washing instructions. It is recommended to wash at 60 and try to wash other items too as you may not like to run the machine too empty.
Once you are done with washing the jumpers, let them dry completely before using them for your craft projects. You can use them to make baby booties, mobile phone pouch, mats and many other things.
Occasionally, it takes several washes to felt the material completely. You can tell it's felted if you cut into the jumper and it doesn't unravel. The material will feel thicker than before and there may be some shrinkage.