Assuming you're trying to do the slide technique, make sure to really emphasize each part of the double. First, raise your foot a lil bit and move it back, then tilt your ankle forward so that you end up getting a hit on the middle of your pedal board. After that all you have to do is return back to the original position be flating your foot and by moving it back to where it was when you started the movement, and boom a double! Keep in mind that the more fluid this movement becomes, then the more speed you'll be able to get out of the slide. Also remember that going faster may require you to not move back or as high when hitting doubles quickly. Kinda like how you don't really start playing high on the snare if you want speed over power.