0. No miniskirts. Must be at least knee length.
1. Solid, muted colors. Black, Navy Blue, Green, Tan, etc.
2. Symmetric, unadorned hemline
3. No slits (except for a short one at the back)
4. Thick enough to be opaque
5. Goes well with a unisex T-shirt/polo shirt. Bonus if you've seen a woman wear the skirt with a unisex T-shirt or polo shirt.
6. Bonus if it has belt loops that can fit a man's belt
7. Bonus if it's twill, has a zipper fly, snap brass button, and decent-sized pockets
8. Wear skirt with a fitted unisex Tee/polo shirt and male/unisex shoes.
9. Refrain from wearing any cosmetics/piercings while wearing skirt
10. Make sure skirt is only "femme" thing about your entire outfit
I even would add some points:
11. No dress
12. No tights or pantyhose
13. No high heels or knee high boots
As I said, I don't worry about looking masculine at a casual glance, but it has taken me quite a long time to explore how comfortable I am breaking these "rules". I now wear miniskirts (rule 0), but I started at knee (kilt) length and went longer before trying anything shorter. At the start, I actually stood with a mirror and experimented with adding and subtracting elements to see how feminine I was prepared to go. Plain sober colours like navy blue and dark brown were a safe start (rule 1), though I have, of course, been wearing tartans for decades, and I have branched out into a few select colours and patterns, usually going for geometric or abstract designs in only one or two additional colours. I generally avoid embellished hemlines (in sleeves as well as in skirts), as I think they look too feminine, but I would wear an asymmetric hem (rule 2). I more or less agree with the restriction on slits (rule 3), though I tolerate a short front slit. I have some skirts which are not quite opaque (rule 4): I don't mind people seeing that I do in fact have legs. Although in summer I tend to wear T shirts (rules 5,

, I have frequently worn a man's buttoned and collared shirt with some of my skirts, and it really doesn't cause a visual problem (any awkwardness of transition is nearly covered by a waistcoat/vest). A mandarin or grandad shirt with no collar can also work OK. Yes, belts are good (rule 6). Not sure about twill, but the rest of rule 7 is fine. I'm too old to make much use of cosmetics and I don't do piercings (rule 9); but when I first wore skirts (or even tartan kilts) I was very inhibited about wearing my longish hair loose, or adding anything else femme (rule 10). However, knee high boots (rule 13) were alongside skirts as things I felt unreasonably deprived of and was determined to wear. (They are in fact very useful for the relatively masculine look, as they avoid the narrowness at the ankle below a skirt hem which can give a rather feminine impression.) Except in summer shorts season, I find that tights (rule 12) with any skirt above the knee are generally preferable to exhibiting my hairy legs (which I can't shave as my wife would object). And I am gradually working out which dresses can be made to work with a male torso, often combining them with a man's jacket or coat, and wearing some of them over collared or collarless shirts depending on neckline finish.
Obviously I'm now happy with a certain degree of subversion of the gender binary, to the point where some might regard me as cross-dressing, though I never make any attempt to disguise my sex. I still avoid pink, and most kinds of frill or scalloping. The "rules" would indeed be a guide for anyone trying to wear skirts with out breaking too far out of the Man Box, but I'm happy that I've managed to step out a bit further than that. This does mean that I'm probably not a good role model for the "manly man" approach to skirt-wearing.