(For the last model I completed, the bowsprit and the masts below deck, were to be octagonal. Cutting 8 equal faces on round dowel wasn't so bad, but cutting the bowsprit from a square was a problem. The kit instructions for this were laughable, and my free-hand attempts were unacceptable. So I did a little geometric doodling and devised the method described below.)
Step 1: Mark each longitudinal face (all 4 of them) of the piece with two parallel line drawn as follows

Where 3/10 W just means 3/10ths of the dimension of the square cross-section. So the marked piece looks like

Step 2: Then trim off using the guide lines on adjacent faces.

To give a specific example, the stock piece supplied for the bowsprit had a 3/8" square cross section. My little calculator gives (.3)(3/8)=.1125. This is a smidge less than .125=1/8, so I marked each face as

And when cut and lightly sanded - it really looks octagonal!

Footnote: How close to true regular octagon is the cross-section obtained in this manner? The answer is REAL CLOSE.
Specifically, the marked lines should not be marked 3/10 but rather (2-V2/2=.292893…. which is real close to 3/10. If the piece were 1" square, the error in the face width would be less than .02" and on my bowsprit mentioned above the face width error is less than .006". That is non-detectable even with our modelers' magnifiers.

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