Pressed for time -- please forgive short answers:
1) How to do calendars *properly*??
Whenever I want to think about "properly", I think of CBOE's Russell Rhoades
https://www.interactivebrokers.com/en/index.php?f=2227
He just did a webinar last month, as a matter of fact.
2) How far back? Weekly(s)?
I hate to own. So, I am looking to buy these spreads as cheap as possible -- maybe get paid to write them. So, I'm not writing a month apart, or even a week, but 1-day. Thus, there is an opportunity for ME to write calendars for 0¢-20¢ on the third Thursday/Friday of every month, and add in some special occasions when the month ends on a Tuesday or a Thursday (to take advantage of a Monday or Wednesday or Friday expiry).
Not *proper* by any means, but *damn* cheap.
3) When to close/roll/expire?
I think of these as lottery tickets with sale-back/salvage ability.
I write them *doubting* they will pay (with some pop in volatility)....
Thus, I'll evaluate them as their time winds down, and consider
a) selling the calendar outright
b) turning the calendar into a diagonal
c) turning the calendar into a vertical
If I choose the max[expectedvalue{a,b,c}], I have thus far been profitable.
4) WHERE to write??
I have no idea. I'm just looking for a *really*cheap* way to hold a long position.
I am just learning -- this being "owning" a net position -- something I have assiduously avoided for 10 years.
But *DAMNNNnnnnnnn* with the low vol making option-selling a *ridiculous* enterprise, ....... time to check out the other irons in the fire.
So, I've done this maybe a dozen times, through 2-3 expiries.
I have 4? positions on now, I think they're all 2-fers, I think they're all calls, I think they're all ITM.
Pricing was weird today -- I have drawn no generalities.
(Theory is one thing: this is rubber-hits-the-road, slap-you-in-the-face reality.)
I HATE owning. HATE HATE HATE. But,

, you do what needs doin'.
I would rather write OTM, as that's where my *life* has been, and perhaps more importantly, should I wish to exit, volume is usually better close OTM than ITM.
Just remember that as a debit spread, the risk is established going in, and from there, if you can exit flat_or_for_more, you're golden.
The one, basic, Rule To Rule Them All: write when vol is in the gutter, and not when it's up, cuz you're writing positive-vega trades.