The first thing that you should always do when you are learning how to wash sweaters is looking at the care instructions provided on the tag. Some types of wool are incredibly delicate and can only be dry cleaned or steam cleaned. If the wool can be machine washed, avoid excess friction or any heat that can ruin the wool.
To clean your wool sweater, submerge it in a basin of cool water with just a couple of drops of soap. Swish the sweater gently through the soapy water and allow it to soak for several minutes before you rinse it with cold water. Avoid rubbing, twisting, or wringing the sweater. Instead, gently press it to get rid of excess water and then lay it flat on a dry towel and allow it to air dry.