http://fourhourworkweek.com/2013/05/06/how-to-cut-weight-ufc/
Keep in mind, you won't actually lose 20 lbs as per the thread title by just the simple water load + cut. Treat this more of a guideline.
I use the epsom salt bath(s) to make make via water manipulation, no exercise, I don't want to be heading into the fight tired and muscled cramped up a few days before. The week of the fight is the taper week, there shouldn't be any hard exercise done during that week anyways.
This is my routine during the week of the fight, the only difference is I don't do the liquid meals anymore, it doesn't really matter as long as I'm hitting my daily macros. Carbs and sodium are kept low so I don't hold as much water. 1g of carb holds roughly 4g of water.
I'm heavier than you, and last time I fought (Nov) I was 157 before water manipulation and I cut 10.2lbs to make 147. The water cut itself was 6.6lbs. The rest was cut with an epsom salt bath round* for 3.6lbs (this includes bath and in bed for a short time)
*epsom salt bath: 20min in the bath, immediately I put on my sauna suit, and laid in bed with my blankets + covers on for another 20min as well. Combination of both shaved 3.6lbs off. Its important when you're in bed covered up, that your body temp is still high, and you'll still continue to sweat out. Also put a towel or something on your sheets, so your sweat won't drip down and ruin the mattress.
Also, if you decide to this, the bath takes a bit out of you when you're dehydrated (from the water load + cut). I felt fine in the bath, but as soon as I stood up, I felt like I was kicked with a headkick, and was somewhat disoriented for a good 2min. Have a spotter or someone by in case something extreme happens. You don't exactly want to be passing out in the bath while submerged.
Just remember rehydrating (including recovering your electrolytes) are just as if not more important than water cutting. If you already know this, thats great, if not look to see how.
1L = 1kg (or 2.2lbs). The body absorbs roughly 1L per hour, so if you cut 12lbs, you will need 5.45L so roughly 5.5h to be hydrated, its always better to have more, recovering 5.5L would being just at the minimum to compete safely