This is a technique I pickup at a boatbuilding site a long time ago because long straight lumber is not always easy to find especially if it is sundried which is best for boat building.

First a long straight edge is attached to the lumber along the edge you want to straighten:

The straightedge should be recessed away from the edge of the lumber so the entire length of the wood's edge is exposed. I like using drywall screws for attaching the straight edge. If having screw holes is an issue, one could also use nails or even build a sled this would be much more involved.

Next a "spacer" is clamped to the fence about 1mm higher than the lumber to be straightened. The spacer should also be at least as thick as the kerf of the blade.
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In this case, I used a piece of 5mm plywood. When the curve in the wood is large, a thicker spacer is more efficient. The fence is then positioned so that the exposed side of the spacer aligns with the cutting edge away from the fence.

The idea is simply that the straightedge will ride on the spacer allowing the saw blade to cut the curve out of the workpiece.

Sometimes several passes are need and this is ok. Below are pieces that were cut off.

Using a saw guide is another way of straightening but it requires that the workpiece be a bit wider. With this technique the lumber can be even as narrow (or maybe narrower than the straightedge.

The same technique also works with a router table by the way.