I was going through your double diagonals and was not able to glean guidelines as to how to open and manage the trade.
For example, when (how many days out from your long/short strike expiration) do you put on the diagonal, how do you choose the short strike in the near month (your own criteria, if you will please!) and I read somewhere in your blog that you cover them when the delta of your long exceeds the delta of your short (making it more vulnerable to price moves against that leg).
Also when you do take off one leg of the diagonal do you roll up the other leg (since it is very likely that it is now quite far away from the current price).
You are asking for a whole lesson on how to trade a specific strategy, and that is an entire book chapter in itself.
Double diagonals (DD) are managed similarly to iron condors (IC), but there are enough differences to warrant additional discussion.
First, take a look at these posts that I wrote for InvestorPlace, formerly known as the OptionsZone. They provide an introduction.
Next, review Chapter 21, if you own The Rookies Guide to Options.
Then look through the previous posts on double diagonals.
Come back with specific questions.
My criteria are not likely to prove helpful to you because I seldom use this strategy. I only use DD under one condition: To adjust the vega risk of my portfolio:
a) When my portfolio is short too much vega [That means I have too much risk if implied volatility moves higher. Of course, I'd profit if IV moves lower], I add some double diagonal spreads.
b) When I believe implied volatility is 'low' enough so that I don't want to be short any vega, then I reduce my portfolio to vega neutral by trading both iron condors an double diagonals.
c) When I believe that implied volatility is so low that I want to bet that it will increase soon, I'll trade double diagonals almost exclusively.
I consider this strategy to be very similar to iron condors (it is an iron condor with an embedded calendar spread), and usually prefer to trade the simpler to manage iron condor.
If you want guidelines for entering positions, I suggest picking (not trading, just mentally selecting) the calendar spreads you want to own and then trade the double diagonal accordingly.
My preference is to open the double diagonal when the shorter-term option is about two months from expiration and the other is one additional month.