To connect MGN 12 to a 3030 extrusion at 90 degrees. Very tight fit, you will need to use mallet to tap it in with. Really does not need screws but you can use them if you want. M4 screw and nut for top and wood screws for the bottom screws.
If too tight use sketch up file to shave down to you needs. You will need to mirror for left size, I suggest Microsoft 3d Builder. It will also make any export solid.
Please note all files at scaled at 10x. So, scaled down.
For complete Linear motion see Linear motion guide 3030 by Lehaiver