Okay so my wife has some really nice looking barn board (9 ft x 13 inches x 1.5 inches)
The boards are flat but the sides are wavy. It's like a tree was fed through a band saw and they didn't bother to make the boards proper rectangles.
If I want them to sit flush side by side I will need to cut that.
My understanding is that if you don't have a table saw you can measure a straight line, clamp down a straight piece of wood to guide your circular saw.
But if the board is 9 feet long how do I clamp it securely? How many clamps do I need?
What if I don't have a piece of wood that is 9 feet long to guide me?
Just take it to a lumber place and they'll usually cut it for you for a fee. I had some ply board I needed cut and not enough space to do it so I took it to a lumber yard and they charged me $5 a cut to trim it to size.
Otherwise you'll need a size able straight edge and clamping it if the board is wider than a foot will be quite difficult as you'll have to clamp it on the side opposite to where you'll be cutting.