Use or hire a vax style carpet and upholstery shampooer. I have one in the house, I got one years ago and when it died bought another, even with full leather seats on all our cars, they are stuill great for carpets etc.
And the washng powder tip is a good one, I use regular DAZ in my shampooer, it is low foam and really gets the crud out. Rinse well using the shampooer with clean water on alcantara, let the alcantara get nice and wet and dont be too "scrubby" with the nozzle, if the nozzle is old or worn it can pluck the alcantara a bit, take your time and go over them a few times, also use a nail brush on the solution to scrub then up a bit before you wet vac off.
BTW, do an inconspicuous area of the alcantara with the solution frst, just o ensure it is nto going to run! !!