I'm a fan of Sid and what he does, but am also aware of the distortion and exaggeration of the look by many on the internet who try and emulate it. Here is my take on the matter.
The jacket will have to be a sport coat, not a blazer. If you're wearing it with jeans, keep it informal. Natural shoulders, nothing showy, but at the same time nothing too staid, so as to avoid looking like you have on an orphaned suit jacket. There are some on Sid's site that work, and are very fairly priced when taking into account the quality of the item.
Keep the shirt and tie relatively simple. Button-down or spread collar- knit, repp, or club tie. Plain blue or white is the best choice, with pink an option as well. Knot the tie using a conventional four in hand method, and avoid using the keeper on the back. Both ends should be roughly equal in length, though. An uneven narrow end is best approached in tiny increments and with plenty of caution.
A.P.C. New Standards and Levi's 501's are both acceptable, but must be either rolled (only if selvedge is present) or hemmed. No break. Some proponents of the "Mashburn look" will tend to have them tapered, but in my experience this is not necessary. Regardless of your build, they'll look cleaner and make you look less top-heavy if you refrain from tapering them. The "sand" or "wheat" ones are especially nice, and can serve as either jeans or chinos depending on the situation. They won't draw as much attention to the "jeans and leather dress shoes" combination that encounters so much resistance from the more traditionalist members on this forum.
If you're looking at more formal trousers, it's important to maintain a very balanced approach. Don't wear them too fitted, and certainly don't hem them too high. That being said, keep them relatively slim and avoid a break at all costs. Just make sure the leg is not too short and the cuff, if you choose to wear one, is not too large. 1.75 to 2 inches will do, and it's best to keep it between those two heights.
Belts may be either a plain brown strap and brass or silver buckle (both in a visibly aged state) or one of the profusion of fabric D-ring belts available on the Sid Mashburn website. Don't go too far with these unless the rest of your outfit could be considered quite conservative. The plain navy repp silk one is especially versatile.
As far as shoes are concerned, suede or tan leather penny loafers are the safest choice. Many other styles are acceptable, including brogues (a more English shape, though, is preferred to a shell cordovan gunboat style), plain bluchers, single monks, or the double monks for which Sid's is best known. If you're going to wear double monks, though, and you are not:
A: Sid himself,
B: An employee at the shop or a friend of his, or,
C: An older Italian or Asian gentleman surrounded by street photographers and cigarette smoke,
then please think twice and then once more after that about wearing them with the straps unbuckled. They're beautiful, sleek shoes, and they are meant to fasten and fit in a certain way. It does indeed look kind of cool to undo them and leave them that way, but it can't be everybody's trademark. I suspect it's not the best way to wear your bench-made English footwear, either, if you want them to last. Same thing applies to socks. Sometimes, you truly do need to wear them. You don't need to wear them as much as a lot of people think you do, but you still need to be practical. They look just as good with socks as they do without.
Lastly, don't take it all too seriously. Use elements, but keep it within the bounds of your own style. This is a look adopted by a small group of people, as their own calling card. Don't try and plunge into it headfirst. Any guy could go into that store and emerge with clothes that suited him and left him looking better than most. There's certainly enough to choose from, both in the store and on the site, and whatever you pick for yourself will likely be a good choice.
I guess what I'm trying to say is that although the look you've been seeking is a little on the extreme side, you've certainly chosen a good source from which to derive stylistic inspiration. Don't adhere too closely to an archetype; just find out exactly what works for you about this kind of setup and work from there.
Sorry if I've carried on a little. I've been meaning to put in my two cents on this particular style for a while now.
I'd be interested in hearing what some other members think, as well.
Best of luck in your sartorial endeavors.