A couple of ways to do it, depending on how much you have, what you want to remove, etc.
I have a maxi jet with a short piece of hose on the intake that I use as a vacuum. This can't handle much without getting clogged, though, so you have to take it slow. You can just hang a filter sock in the sump and have the discharge hose from the pump go into the filter sock to catch the detritus.
If that doesn't work for you then the bucket head or a shopvac will also work. They will suck higher volumes and empty the sump pretty quickly. Personally, I need to do it pretty rarely, so I'd rather deal with the limitations of the maxi jet that I already have over getting another piece of equipment to use only occasionally and store the rest of the time.
Another option is to simply pump all the water out and scrape it up with a plastic spatula or something similar.