They are not that hard to compress with your hand. If you can install one end first without cutting the strap first great, but after cutting the strap (for smaller packaging) you can collapse the shock by pressing one end on the concrete floor. Install into hole before the shock fully extends. If it fully extends on one end just push with your hand until it fits. The gas there is not to hold the car up but to help control the oil from foaming.

Or just put the jacks under the frame so the suspension is extended and use a jack like ‘Knowsnothing’ stated.

Note you can also collapse the shock by hand and remove the strap without cutting it. This way if it’s too hard for you to overcome while under the car you can try putting the strap on and cut it while under the car.

It’s one of those things where as soon as you start doing the work you wonder why you asked the question to start with.