Here's how I do it:
1) Before getting started
- Create a jig or determine some other means of aligning the mirror into the desired position on the hub and stay in that position while the silicone cures.
- Be aware that the mirror surface is quite delicate and can scratch easy, even setting on something as benign as a piece of paper - I like to set up my jig so that the mirror will be mirror side up such that it never has to rest on anything.
- Find / create three "spacers" to hold the mirror & hub apart by the desired distance while the silicone cures. I use small lengths (about 1/2 the secondary minor dimension) of a .017" guitar string
- Get a fresh tube of 100% silicone for the job (yes, it goes bad). I suspect being fresh is more important than the brand - I did my first secondary some 20 years ago, and it's still holding strong.
- Get some cotton balls (optional if not intending to clean the mirror surface)
3) Clean the two surfaces with a little soap in some warm water, then just warm water to remove the soap. If not intending to clean the whole mirror, Q-tips could be used to do a more selective cleaning of the back of the mirror only.
4) Rinse with isopropyl alcohol. Cotton balls good for gently wiping away drops then letting it evaporate (which happens quickly). Extra gentle on the mirror surface, if doing that.
5) Holding the mirror by it's sides by hand, apply 3 "dabs" of silicon to the back of the mirror, I think the rule of thumb is 40% of the way from the center to the edge for best support , though I tend to go with somewhat wider spacing, on the theory that I'm seeking a balance between the traditional concern of proper mirror support and, particularly on faster scopes needing with tight collimation requirements, lowering the (albeit tiny) collimation error resulting from the shifting weight on the silicon pads themselves (silicon has a very low Young's modulus, i.e. it's (intentionally) "stretchy"). It's this same reasoning that leads me to use the somewhat smaller .017" gaps than is typically recommended.
6) Put the spacers in position. With the little pieces of music wire, I put the tip of each piece just inside the silicon pads, which keeps them in place (and they're easy to pull out once cured)
7) Set mirror & hub together per alignment / jig.
8) Place in a relatively warm location in the house, in a place it will not be disturbed for 24 hours (no cheating).
9) Pull out the spacers
As for primary, I've not liked the idea of having clips against my primary mirror, but would also be horrified if it were to fall out should the scope be titled to the horizon. After the mirror cell & side support is complete, I'll securely attach a couple of short nylon web straps to the mirror cell on each side of the mirror, reaching up along the side of the mirror (but not extending past its surface), and then silicone the straps to the side of the mirror, leaving a bit of slack in them (in an effort to not apply any force to the mirror).