Making some progress slowly...

I have managed to generate a map (.img) from one of the shape files or layers.
I used a road layer with only polylines in order to learn how the process works.

If someone else works with this map, please add your info.
This is how I have done so far:

1 - : Transform the SWEREF99 projection to WGS84 with OGR2GUI64 program. Works fine, single files or whole folder can be transformed. WGS84 is 4326 (ESPG:4326) and should be given in the projection field, not obvious for those of us not working with these issues...

2 - : With DBF Commander I open the corresponding .dbf file. Change the record layout so that the fields are named "name" and "grmn_type" which seems to be the names MPC wants as can be seen in the sample files from MPC. (?) Seems as the length and type of the fields can be changed without problems (?)

3 - : With DBF Commander change the values in "name" from the numeric code from the source to characters and change the numbers given in my source files to a name that will show in the map, Change the values in the grmn_type to names that MPC will accept, I found the acceptable names from MPC "Edit Feature Display Properties" "Garmin Lines" or "Custom Lines". I chosed to map each of the roads I wanted to be used to "CUSTOMIZABLE_LINE_X". Find and replace function of DBF Commander works fine but takes a lot of time since there are many (over 100 000 vectors just for the roads in my district) records to search and replace.

4 - : With MPC give a product name, select a build folder location and add map/shape file(s).
Configure map builds and Edit Feature Display Styles. Here I can see the acceptable Feature Types and also edit the lines so that I can have the line that I want (this can later be changed with TypViewer).

5 - : In MPC "Product Configuration" tab "Detailed" uncheck "Enable Route Calculation" since I do not want a routable map because the roads and especially the foot-paths are not connected always and any routing will produce bad results.

6 - : In MPC "Product Configuration" tab "Install" check the "Create Product Image" so that a gmapsupp.img is produced that I can copy to an usb stick folder garmin and see the map in BaseCamp for quick feedback of the result.

7 - : Build map. This takes a lot of time also with the "easy" file I have tested with so I suppose it will need overnight compilation for the larger ones.

The generated gmapsupp.img looks just as expected and I can also play with TypViewer to change the apperence of the map so far so good. I will try with a layer with polygons next and hopefully learn some more and read help files again, however some questions needs to be solved.

There are 36 layers in this map and each layer has many types that needs to be mapped to "grmn_type", this must be made manually in DBF Commander and will take a lot of time but it is a one time job so OK.

I suppose that I can generate one .img file for each layer and the join them in GmapTool, generating all 36 layers in one batch of MPC seems heavy but maybe it will work...

All generated .img files will initially have the same FID but I assume that I can handle that with GmapTool.

Some symbols have an angle of direction defined, have not found out how to specify this in "Garmin style" yet.

Have not tested the generated map in a gps yet.

To be continued...