There probably are some sites that show how to make trim but I don't know who they are - but......
Having made some fairly complex custom moldings in the past, I've found that with a little ingenuity, some head-scratching and up-front planning, it can be done - and done safely. I'm assuming you have the basic tools to mill the stock and tools to shape the trim such as router table with a solid fence and a few router bits (or a shaper), a tablesaw and a decent blade, some Bondo, and a way to cut/file a piece of card-scrapper stock into a template to make a custom scrapper (if needed). I use a Dremel tool and diamond coated bits to help shape the template if it's real intricate. Not needed for most molding profiles I've done though.
There are several sites that have molding profiles on-line that you can download and print out. The magazine sites (AW and FWW) have several How-To articles and Methods of Work type articles that show how to make cove moldings using a TS and an angled fence. For other moldings (not the large cove moldings) the router is what I use. A shaper and custom made blades are another way if you have the spare change. See Tom Plamann's site for some examples
Here's what I've done. First get a couple of the best you can find, rift cut, 2x4's - yes 2x4's. Let them acclimate to your shop for a week then cut them to the length plus a couple of inches for the longest piece of molding you expect to make. For a small shop like mine (12'x20') and full of machinery, a 7' long piece is about all I can handle which is long enough usually for most doors and windows, cabinetry etc. I use 2x4's because they're lighter and less expensive than making a 7' carrier out of laminating up some hardwoods or MDF pieces. You'll appreciate having the stability of a wide edge to run your stock vertically past a router bit and manipulate the piece easily while you work it.
The 2x4's are milled to make them into 1-1/4"w x 3" - flat and square carriers. Square meaning the edges are perpendicular to the faces. A slight bow in the length is okay since the molding stock gets attached to the carrier and a slight bow will not affect the shaping of the stock along it's length. No cupping or twist allowed - that will affect the shape of the molding. Place the 2x4 on edge, attach the molding stock with double-back tape with the edge of the molding even with edge of carrier - the reference edge.
While these 2x4's are acclimating - order the router bit extender from MLCS ($25). When using a router table, you will need to extend a router bit at times to make a molding shape and you'll be tempted to just pull the router bit up a little -creating a potential safety hazard. The MLCS extender gives 2-1/2" reach if needed. I've only needed an additional 1" on any of the moldings I've made so it works out nicely and makes it much safer in my estimation.
I attach the molding stock to a length of milled 2x4 (typically either 7' long or 3' long) with double backed tape going the full length - not just short sections. You want the molding flat to the carrier and any gaps where there is not tape will telegraph thru after routing. The 2x4 carrier now gives a solid and safe way to mill the molding. Since most window and frame moldings have a thin edge, they would be difficult to handle by themselves and route vertically. The 2x4 carrier makes routing the moldings far easier and safer. I can now concentrate on "How do I make that profile" and less on "How do I keep 10 fingers attached".
I've found that only a few basic router bits are necessary for most molding shapes - different size round-nose bits, a table-edge bit or two, dish carving bit and a couple different sizes of half-round bits. Obviously some shapes just can't be done on a router setup unless you have a vertical bit that matches the shape you want such as a vertical panel raising bit. I have a multi-profile bit and while it sounded like a nice bit to have, I have not found a use for it - yet. So save your money unless it really fits what you want to do.
This site
If you already have a molding that you want to copy (and depending on it's complexity), make a mold of it using the Bondo. Once the Bondo has cured, attach some self-adhesive sandpaper to it to work down to the final shape of the molding you're making. If the molding is intricate and has some sharp edges that need shaping, I've made a custom scrappers to do the final shaping. No... you wouldn't want to use either of these methods for making a lot of moldings such as a whole house full of rooms. But if it's for a cabinet decoration or picture frame, then it's probably worth the effort.
My sister-in-law wanted some extra-wide window moldings for a bay window they installed in their log home. The profile of the molding she found and wanted reflected the "log look" but it was a non-standard size and had to be custom ordered at $15/ft. Not all that complex in design really but just enough to be a nice challenge. I was able to duplicate it using 3 router bits, the extender and a Bondo mold to do the finish sanding. I first made a test section (~24" long) to work out how the cuts would be made and in which order. Once I was comfortable with each test cut, I marked the test piece with notes and numbers that indicated the sequence of cuts. The test piece itself then became my reference piece for setting the depth of cut for both the height of the router bit and depth of cut by moving the router fence in or out.
I use a Jessem Rout-R-Lift
I know the above doesn't directly answer your question and maybe someone else will - like the Duke of URL's if he's lurking about but there are a number of ways to make moldings. This is what I came up with and its worked well for the moldings I've made. I too will be interested in reading how others tackle this task.
Bob S.