I have read that stock that is not square is useless, and for the most part I think that is true, so I try to always make sure the stock is straight and square, if it isn't, nothing seems to fit right, the tools I use for this are fairly simple, a couple of planes (wooden scrub plane, a #5 jack plane ( mine is a record...a tuned up new model) and a very old #7(mine is a Stanley) and of course a square, the marking gauge comes in later for thicknessing the stock, and the winding sticks maybe the most critical part of the whole job, even though they are the simplest tools shown, here they serve as both winding sticks and straight edges without them,  the whole job would go badly....

The stock, here at left is for example only, but it is part of my workbench project, and is typical of the stock I would normally begin with, this being reclaimed from a shipping container...I would start by using a "lesser" plane that I didn't mind abusing, the dirt and grit found on timbers like these are hard on the cutting edges and bearing surfaces of all your tools, but once the surface has been cleaned, it all goes well.

Here the stock has had one face cleaned and flattened using the scrub plane and the #5, the winding sticks show the surface has a bit of twist, notice how the tops of the sticks are out of alignment, to solve this, you simply take shavings from the high corners until the sticks are aligned.

Notice now the tops of the winding sticks are parallel, this indicates that the surface in question is not twisted and is most likely flat...although it may still be bowed or hollow, use the winding sticks as straightedges to check for that.

Here it can be seen that the stock is now, seriously out-of-square, you could turn to a jointer at this point,  but...since I don't have one, I guess I will have to do it by hand....

I begin with a scrub plane, it has an iron with a serious radius ground into it, and it takes a heavy coarse shaving, I think once I read it was between a smoothing plane and a hatchet, and there is no question, it will remove waste in a hurry. Work the plane diagonally like the photo at the left, working the high side down until you get close enough to use the #5 or #7, keep at it, checking often with the square until the square shows you that the sides are indeed square.

Now you have two adjacent surfaces that are square, two to go.....