The first thing to do is to visit the Classic Hand Tools website and obtain a copy of 'The Essential Woodworker' by Robert Wearing. It's a very well written and well illustrated guide to the basics of hand tool woodworking, and worth every penny. It covers the basics of planing up stock, including how to get faces and edges square and parallel. Link here -
https://www.classichandtools.com/acatal ... Press.html
The procedure with a piece of rough sawn timber is first, plane one face flat and true. Next, plane one edge straight, and square to the planed face. Next, set a marking gauge to the width required, and using the edge just planed as the reference to run the stock of the gauge against, mark a line. It's best to do this in three or four passes, the first very light, the next a bit heavier, and so on until you have a nice mark. That's because most gauges want to follow the grain rather than parallel to the edge, so lightly defining the line first helps to overcome that tendency - the gauge is much easier to control when used lightly. Also, keep a good pressure against the stock, to keep it in contact with the reference edge. Once the mark is made, you can run a pencil down it make it a bit more visible if you see fit. Then, put the workpiece back in the vice, and plane off the waste, taking more care about straightness, parallel and squareness to the flat face the nearer you get to the mark. Lastly, set the gauge to thickness, mark all round, and bring the other face down to the mark - work on the highest bits first, and again, care about straight and flat the nearer you get to the mark.
Hope that helps a bit!