You can put it in a big bowl or tub, wet it really well. Let it sit for a few hours and scoop out the big floaters. Do this over and over a few times then let the soil dry a bit. Wet it again the next day, let it dry a bit more and wet one more time to dry a bit. It will take about a week if you put it outside to dry quick but by the time you do this it will be absolutely fine with little floaters by the time you are done. I did this and then mixed in half and half soil/floramax that I had left over and it's working out fine.
Before i read to do the above, we bought soil not realizing mix was better and so have a base of soil in 4 of our tanks. They have no problems, grow plants perfectly except I think they could maybe use a few root tabs for the swords I have, everything else grows great. Will bite the bullet soon and buy some root tabs but which ones to buy confuse me. Still to new to the fertilizer stuff so what should be in it makes my brain hurt.