A cheap, low-capital (few and cheap tools), and fairly easy to build knife sharpening jig which breaks down pretty well for storage.

This is NOT the first knife sharpener on Instructables, but is intended to fill what I perceived as a gap between the light-weight sharpeners and the heavy-duty beasts. (see last step: "See also" if those sound more like what you are interested in).

As such, it's a large guide-rod model to make heavy use, such as grinding back-bevels, reasonably easy, but it does light work just fine too.

Ease of manufacture and low cost was a high priority during design, and I feel I was pretty successful in that regard. You'll need:

Drill (with something like 3/32" and 1/2" bits)
Saw
Vice
Clamps

Furring Strip (or similar small dimensional lumber)
1/2" dowel rod. (3/8" is way too small. 7/16" is pushing it. 1/2" is plenty stiff)
1" PVC pipe (could be smaller, but I happened to have it)
1" PVC coupler
1 1/4" pipe straps (2)
Steel electrical box cover plate
A bit (maybe 4-6" ?) of steel wire
4 wood screws
2 sheet metal screws
wood glue
hot glue

The principal disadvantage is that the wooden stone holders/guide rod make it difficult to rinse the grit-sludge off the stones - I've switched to doing it in the sink, rather than using a bucket of water.

Second disadvantage is that the angular precision is, at best, only something like +/- 0.7° between different stones, even after calibration. This could be improved with more machine-tool-like construction techniques (ie, drill press, thickness planer), but I didn't have access to such things and don't know how much they'd help...

But enough about design philosophy and trade-offs - onwards to implementation!

Step 1: Guide Pin

Bend a bit of steel wire such that it has an oblong opening somewhat more than 1/2" tall. This is our guide pin: it fits in the holes we'll drill in the post and controls the location of our guide rod.

If the opening is a bit off-center, that's ok - perhaps optimal, as will be explained later, in the "Use" section.

Step 2: Fabricate Base and Post

The base is as simple as it looks. Some design points:

Length should be something reasonable for you. For instance, if you will be using stones as long as 12", you will want to make the thing at least (12+3+2) = 17" long. A bit of slack is good, so mine is 20.5" long from one end to the other.
Width should be something reasonable. There's no point in making it skinnier than the cover plate (about 5"), and a bunch of reason to make it a bit wider, so mine is 7" wide.

Construction of the base is pretty easy:

Cut the wood to length.
Glue it up
Once the glue dries, attach the coupling with the pipe straps.
Fill the gaps around the pipe straps with hot glue to prevent wiggles.
Add a screw to prevent the coupling from rotating.
Heat up the steel cover plate with a propane torch, apply hot glue to the wood surface, and press the cover plate onto the wood.

That done, the post is simply a piece of pipe with holes drilled in appropriate locations.
A screw controls the depth that it can be inserted into the coupling so it doesn't get really stuck and so that it returns to the same location every time.

The height of the hole center locations can be found, approximately, using the equation given in the diagram.

Step 3: Stone Holders

To make the stone holder to guide rod couplings, drill a 1/2" hole as close to parallel to the center of the furring strip as possible, with particular emphasis on keeping it in the same plane as the wide faces.

Just going by eye, I was able to hold about +/- 1 degree, which was good enough for my purposes, but if you need better accuracy than that, go hunt down a drill press, dowel-drilling-jig, or such.

After checking that you did indeed drill an accurate, high quality hole, these are used to make the stone holders.
The only important thing is that the distance from the guide rod hole to the bottom of the stone be consistent across your line of holders; otherwise, a wide range of designs is possible - I've included a picture of mine. In general, the smaller the stone, the lazier you can be. For instance, the holder for my 12" stone has a steel tongue which holds the stone up into a foam pad, while the holders for my 6" stones simply fit the stones' length closely.

Step 4: Use

Since we have a couple tolerances which could stack up to a couple degrees, you'll need to zero your stones. Simply press them flat against the knife-support table (steel cover plate) and note what sort of offset you'll need to use at the post.

That is, if running the pin "upside down" and in the 1° hole puts the stone flat against the table, then to grind a 25° angle, put the pin in the 26° hole upside down.

Clamp the jig to a bench, or such, and grind away. You can easily clamp the knife to the support table if you want.

Step 5: See Also

Perhaps you'd like something smaller and lighter?
https://www.instructables.com/id/Knife-sharpening-jig/

Smaller, lighter, with somewhat less versatility?
https://www.instructables.com/id/Lets-build-a-fixed-angle-knive-sharpener/

Something heavier, almost a Tormek clone?
https://www.instructables.com/id/Homemade-pocket-knife-sharpener-2/

Perhaps you don't want to buy a bunch of expensive stones?
https://www.instructables.com/id/DIY-Knife-sharpening-kit/