Washing is straightforward, and your topper will look better for it. It does not need washing anywhere near as often as your own hair — every fifteen to twenty wears is a reasonable guide, or sooner if you have been using a lot of product.
Use lukewarm water and a sulphate free shampoo. Work it gently through the hair in the direction it falls rather than scrubbing, and take care around the base where the hair is knotted in. Condition the mid lengths and ends, keeping conditioner away from the base itself.
Rinse thoroughly, squeeze the water out rather than wringing, then lay it on a towel to take out the excess. Air dry on a stand where possible. Comb through gently with a wide tooth comb, starting at the ends and working up.
EDIT ME: name and link the shampoo and conditioner you recommend — you stock hair care already and this is the right place for it.