with a sharp wide wood chisel or a paint scraper that has some rigidity.
Also, on outdoor carpet, especially with a good thick rubber backing, slight imperfections or stuff stuck on the slab will not come through or show as in indoor flooring.
I would scrape it clean, hose it down, let it dry completely, sweep it down several times, then apply the outdoor mastic, going precisely by directions.
I always dry measure and cut the carpet and have it already fitted and ready to go. Then I apply the mastic and wait for it to dry to a tacky state. Then grab the carpet and on something small like this, I would set one half of the carpet, then change positions on the carpet and lay the other half. Smooth it out and you have it.
I would not do this in the late afternoon. I woudl do this early in the morning so that the mastic does set and cure before nightfall.