I don't mean to be rude but this question has been answered a gazillion times on this and the stone forum so you may want to search a bit. I will say this though: you will find many different answers, confusing names and even conflicting advice so my advice is to go with local knowledge. Find a patio that you like and ask how it was done, visit the place where you intend to buy your material and talk to people.

This is how I go about patio prep

1)excavate down paver thickness plus +- 7" (for 6" gravel base)
2)apply geo textile fabric
3)add 6" of 0-3/4" 'cheap crushed' gravel base (4" is acceptable for a patio, 6" is better especially in Denver climate) seek recycled crushed gravel if available.
4)tamp gravel base with a vibrating plate over two layers (at 3" and 6")
5)top up low spots to make level grade (sloped slightly for drainage in appropriate direction) tamp.
6)apply 1" layer of sand or limestone screening. sand is preferable for flagstone as it shapes to irregular underside of flagstones and drains slightly better.
7)level off perfect grade playing with a 4' carpenters level or leveled pipes and 2 by 4. slope 1" over 4 ft in appropriate direction for drainage
8)apply pavers (tightly) or flagstone
9)sweep in polymer sand to fill in joints
10)tamp, reapply polymer to top up joints, moisten

Alternative bases for flagstone-
1)geo textile fabric
2)4" crushed gravel (0-3/4") base
3)4"- 6" Concrete pad with rebar
4)apply thin set mortar with 1/8" notched trowel
5)place flagstone
6)apply mortar to joints, have sponge handy

Least thorough option for thick flagstones (2" +)
1)excavate slightly more than 3"
2)apply 1" sand for levelling
3)drop flagstones in place, hammer down with deadblow mallet
4)fill joints with polymer sand, moisten

Hope that helps.

Thanks natural-sense!

What's the benefit of the 6" of gravel? Everything I've read has suggested 2".

A thicker base is beneficial for a few reasons. First, over the long run the deeper and better compacted base will hold up to the traffic and weight that will be traveling on it. Driveways for example require a 2' base to support the constant weight of vehicles. Although this is of lesser concern for patios where load bearing is far less is cant hurt and in the long run can save you from having to re-level low spots.

More importantly however is the drainage issue. In Denver obviously you are susceptible to freeze and thaw cycles. A gravel base is permeable allowing water to percolate down and thus reducing the heaving damage by taking it further from the surface where it meets with clay and/or silt. The deeper the better as you want to keep water collection as far down from the surface as possible. It also acts as an insulator and reduces the freeze susceptibility to a deeper level.

Lastly, a gravel base is flexible. To a degree, it can absorb some of the forces nature exerts on your sub grade. The net result is less damage to your surface level.

6" is definitely on the safe side of things, but thats the route I offer for my clients in Zone 4.

Most important however on top of everything is to slope it off so that there is less standing water in the first place. 1" slope over 4' should do ya fine.

enjoy!