Hi Kestutis. Welcome to the site. I’m obviously not Paul, but you’ll find the members here are a wealth of information (I can’t promise I will be, though :)).

As they say in these parts, “hold your horses.” By your picture, looks like you’re dealing with a plane that is indeed not-square. But the funny thing I’ve found about squareness is that it’s more of a cultural ideal than a pragmatic necessity. First thing you should do is see if, with a sharp blade and clear mind, you can compensate for that “out of squareness” simply by adjusting the lateral lever. In other words, purposely skew the blade in the plane to counteract the tilt. Shoot the end of an appropriately thick board, and then check THE BOARD for square, not the plane.

At the end of the day, I think you’ll find obsession over squareness and flatness are more distraction than anything else. I have a 120 year old No. 8 that has a large belly in the sole. But darned if that fella doesn’t make the prettiest, flattest, straightest board you ever did see.

Now, you may not be able to make up for the skew that way. In which case, get hold of a flat slab of granite or other stone (or float glass), procure some decent course sandpaper, lay the paper down on the stone, and get to rubbing that plane across that paper until the cows come home. Keep the plane assembled. Make sure the blade isn’t sticking out. Use a sharpie to draw diagonal lines on the sole bottom occasionally to check your progress. This is a tool restorer’s right of passage.