How do I adjust the rear suspension on a 2010 Honda Stateline to create a softer ride?

Mostly the handbook is the best source for that information.

Traditionally the front suspension of a motorcycle tends not to be adjustable.

Traditionally the rear suspension tends to be adjustable only as far as increasing the ride height a little to compensate for carrying a passenger.

Traditionally if a different spring rate is desired or firmer or softer damping or even adjustable damping is wanted these must be fitted in the aftermarket.

Some modern bikes have adjustable (and programmable) damping from the handlebar computer but again reading the vehicle owner's handbook is the way forward.

Generally you don't adjust suspension on motorcycles for the height of the rider. You make suspension adjustments for the weight of a rider by making it softer or firmer.
You can lower the front by moving forks up through the triple trees and the rear by using shorter shocks. But by doing this you are changing the steering geometry and suspension travel that has been designed for your bike.
If you already know how to ride a motorcycle you are better of working out a way to mount the bike with out lowering it. If you are learning how to ride, buy or borrow another bike until you have learnt the basics.
I have seen children racing motorcycles that use blocks or have people hold them up for the start.

You Raise the Center of the Bike, so the Rear Wheel is OFF the ground and then there is a Collar that screws up and down on the Spring/Shock. Increase length softer Ride, Shorten Length for a Firmer Ride.

Attempted to send solution and it didn't go through. I had the same problem with my Goldwing. Believe it or not I found the help I needed on Youtube. Type in rear tire removal, year make model. Not only will you find instructions there will be video's of the entire process. If you have a laptop its a bit better so you can work on your bike while watching the replays. Hope this helps

Pull off the plastic left side cover below the seat - go easy so you don't damage the tabs.
Remove the tool box cover.
Remove the tool box tray (2 bolts).
Use the tool provided in your OEM tool kit, and use the flattened tubular extension for more leverage.
Use gloves to save your knuckles.
Higher # stiffer - lower # softer.
A lot easier if you have a jack to get the weight off the rear wheel.

try adjusting the preload on the shocks to a softer setting.