For designing my schematics and the PCBs layout I use KiCAD from CERN. Use whatever software / online tool you like.

For plotting / exporting the design I set the following parameters (File -> plot):

- Only export F.Cu and B.Cu, so front and back side copper, you do not need more. For one layer PCBs only export F.Cu.

- Plot as SVG

- Drill holes 'small' so you have a center point for drilling

- Mirror the design

The following steps are only nessesary if you are making two layer PCBs. For one layer simply print out the mirrored design and center it on the PCB raw material. Then continue with 'Exposure, developing and etching'.

Open the SVG files in Inkscape and align them so that they are on the same height. Flip one side vertically so that the vias / holes are superimposing when you mentally fold one side over the other (vertical hinge). Then draw a rectangle between them with the height of your design and width of your PCB thickness. I use raw PCB material from Bungard which is 1.5 mm thick. Position the front and back layout 3-5 mm apart from the rectangle and then distribute them equally around the center rectangle.

The spacing is important for the etching process. It garantees a good positioning in the holding rails of the etching machine and insures that your design is not covered by the holding mechanism. The next step should not be nessesary but I experienced some trouble when printing direct out of inkscape. One side of the PCB had way thinner trances then the other side although in inkscape the paths where the same width. To avoid this, export a PDF file and print this. You should choose the darkest / uneconomical settings possible to print. The darker the print, the better. If your toner is almost empty the print will be to light and the underlaying photoresist will be developed. I have only tested this with a laser printer, not sure if an ink jet printer will work.

I got the tip that the resolution after exposing is way better with transparency paper than a clear foil, so print on this if available.

Cut out your design and fold it along the edge of the center rectangle so that the printed side is laying on the PCB (Thats the reason one mirrors the design in KiCAD export). Use a metal ruler to get a nice sharp folding in the paper. Take special care that your foldings are straight. Otherwise it will result in a shifted design of the front/back layer of your board and vias / solder pads will not end up superimposed.

Remove the protective foil of the PCB and center it within the layout 'pocket'. Use a nice straight edge of the PCB to align it. Fixate the layout on the PCB with sticky tape and expose both sides (see next step). Now put it quickly into the exposure unit.

Exposure and developing

Wear eye protection when handling the solutions!

I use a exposure unit from Gie-Tech GmbH (sold by Reichelt in Germany) with 4x8 W UV-tubes. With this unit the exposure takes about 130 s per side, but your timings may vary depending on the power of your UV source. Make some tests with small pieces of PCB to find the best exposure time. After exposure the layout is usually not visible on the PCB. Immerse it immediately in the developer solution. I use a sodium hydroxide solution (1 g NaOH / 100 ml water). With this concentration it takes about 2 min to remove the exposed photo film. Make sure everthing is dissolved, otherwise the etching process will not work. Rinse the PCB with cold water and put it immediately to etching. You can use the developer solution multiple times.

Etching

For removing the copper layer at the exposed parts of the layout I use sodium persulfate (200 g/l water) in an etching machine (again Reichelt Germany, Gie-Tech) with optional air bubble column at 45-50 °C. Bubbling air through the solution makes a huge difference in the time it takes to etch the copper away. I have tried it in a flat bowl without air supply and the results were disappointing. The whole process should take about 5 min until you see most of the copper gone. A couple of minutes more and all the exposed copper should have dissolved (about 7-8 min). Do not over etch ! Traces will get very thin and SMD pads smaller. Rather than trying to etch off the last tiny bit of copper of one corner or solder pad while the rest of the PCB is done, you might be better off if you leave it and scrape it off with a knife afterwards.

Wear safty goggles when you drill holes ! A 0.6 or 1 mm dril can break very easily.

When finished, your PCB could look like this or better. Here I used vias with 0.8 mm outer hole diameter. Drill all holes for vias, THT components and sockets preferably with a drill press. The holes here I made free hand with a Dremel tool.

The last step is not strictly necsary but results in nicer looking PCBs: Cut off the extra 3-5 mm PCB at the sides.