It's easier if you jack the back end up, but you can do it on the ground too if you're flexible.
Remember that Softail shocks work opposite then "regular" set-ups. An easy way to remember it is if the adjustment makes the shock longer then the bike sits lower and the ride gets softer. If it shortens the shock then the bike sits higher and the ride gets stiffer.
There are year and model specific instructions for your particular bike in the factory service manual and I would suggest looking at them before you do anything. Most models give a suggested number of turns for a specific weight, but I never used that because you never know who adjusted it before you and where you're starting from. I've always adjusted them by "feel."
"1) Jack the bike up so the rear tire is off the ground. The more clearance you have the easier it is. Don't forget to strap the bike down. You don't want it falling on you while your wrenching underneath it.
2) Loosen the jam nut all the way out. Its a 1-11/16" nut. If it doesn't want to loosen easily (which is often the case), soak it in some penetrating lube.
3) With a spanner wrench (HD p/n 94448-82B) turn the adjuster plate counter-clockwise (so the shock housing moves towards the front of the bike) until its up against the jam nut.
4) Then take a light colored paint pen, or a piece of duck tape, and mark the 6 o'clock position on the adjuster plate to use as a reference point when doing your adjustment. Don't worry if they're not in exactly the same position on both shocks. Thats normal.
Now your ready to adjust the shocks to your weight/load.
5) With the spanner wrench, turn the adjuster plate clockwise no more then 4 complete turns by counting how many times the mark from step 4 crosses the 6 o'clock position. 0 turns is the stiffest setting, and 4 turns is the softest setting.
6) While holding the adjuster plate in place with the spanner wrench, tighten the jam nut up against the adjuster plate by turning the nut clockwise.
6) Now repeat the exact same process on the other shock. The key is to adjust both shocks equally.
The general rule of thumb I use is:
4 turns for load <= 180 lbs
3 turns for 180lbs < load <= 235lbs
2 turns for 235lbs < load <= 300lbs
1 turn for 300lbs < load <= 375lbs
0 turns for 375 < load
235lbs solo rider: 3 full turns
235lbs solo rider + 32lbs pack: 2.5 turns
235lbs rider + 150lbs passenger + 32lbs pack: 0.5 turns
I like a slightly stiffer ride when running solo (3 turns for my 235lbs fat booty), but run a softer ride when my girlies on the back (1 turn). Adjust your shock to taste by 1/4 turn increments till you get it dialed in for you and your riding style.
Note: you may be tempted to skip step 1 and not jack the bike up. Resist the urge. A stock ride height with the bike on its kick-stand, you may be able to use a crowsfoot or similar to get the jam nut loose, but you will have little to no room to use the spanner wrench. So take the time to."