I had a line of insulation decide it no longer wanted to stay attached to the garage rafters. Bought some fresh (and better) stuff. Went to put it up, but my standard stepladder is coming up short (literally). I've got a nice extension ladder, but am unsure if it's safe to use that against the 2x6 beams across the garage? Height may not be enough (how much below the pivot point do you need to stay?) and also concerned about side load on the beam.
A tall step ladder is the best solution.
With an extension ladder, you may find it easier to put the ladder against a side wall of the garage and work on this problem from above. Take some scrap boards that you can lay across the joists to support your weight while you work. Since you're likely on concrete, have someone hold the ladder or use something else to prevent the bottom from sliding out from under you. Resting the ladder against a joist without any nearby cross bracing will result in it bowing and could knock another piece of insulation loose when you remove the ladder.
A quick method to check the slope of the ladder is to place your toes against the feet of the ladder and extend your arms straight out in front of you. Your hands should just touch the ladder. You may also see a diagram on the side of the ladder indicating which side is up and a vertical line when the ladder is in use.
One final note, any walls between your garage and attached home should be drywalled with fire resistant drywall (5/8" type x), taped, and mudded twice. You do not want any path for exhaust fumes from a car to enter your home.