If the piece has a visible care label follow those instructions carefully, making sure not to ruin your new find. In all other instances, hand-washing is recommended to preserve the condition of the piece.
Hand-washing can be done in a clean sink, a bathtub, or a bucket.
In about two gallons of water, wash using just a few drops of detergent or up to a couple tablespoons, depending upon whether your garment is lightly or heavily soiled. As the water discolors, change it out, adding detergent again, but less. Repeat until your wash water remains clear. Swirl the fabric in the water gently while washing, as fibers are weaker when wet.
Rinse very thoroughly, until the rinse water is suds-free—completely clear. Never wring or twist wet fabric to dry it, just gently press water out, then roll in a clean towel to absorb moisture. If the item is sturdy it may be drip dried, but dry your knits and fragile items flat.
There are washing machines that have “hand washing” cycles that may provide you a good substitute for true hand washing, but the genuine process has the added benefit of letting you monitor the situation.