I've never had to do it, but the folks I've seen built blocks they could drive the front wheels up on to, got it as level as they could that way, then just lowered the jacks for stability, not leveling. Your other choice would be to use the jacks to raise the front end higher than needed, put the blocks under, then lower the jacks a bit. In all cases, the rear needs to stay on the ground as they act as your brakes.
2015 Legacy SR340 360RB
2014 Jeep Wrangler Unlimited Toad
Gigi, Poppy and Sadie Lady...On the road, but not full time!