I know Alokin and I use cocreate, I don't know of anyone else. Just try figuring it out yourself first, that's what I did. But, for fun, i'll give you a quick write-up of how to make a 3x3 corner piece.
1. Open up 2d copilot and draw a 19 mm square.
2. Find the "extrude" tool, and extrude the square up 19mm, turning it into a 19 mm cube.
3. Go into workplane, and click new workplane on face, and select a face to create a workplane on.
4. Draw a small slightly curved line in the corner of the face (I don't know the exact measurements)
5. Do this to two adjacent faces, so all of the "drawn-on" corners are touching each other.
6. Extrude the small little corners up (I don't know the measurements) Now you should have a cube, with 3 little prongs sticking out of 1 corner.
7. Fill in the space between the prongs, to make the corner attachment, by creating workplanes, drawing rectangles, and extruding them up. (you must draw the rectangle on your new workplane or else the program has nothing to extrude)
8. Go into "blend", to round off all of the edges. I usually use somewhere between 1mm and 1.5mm for cosmetic edges, and .3-.5 mm for any edges that are part of the mechanism.
Hope this helps, I pretty sure there is a faster way, but this is how I do it. Good luck!