Estes Hex-3, #7263, Build, Part 3, Scoring The Fins

The instructions say to fold the fin lines over a straight edge. I don't see how this could be successful unless you have a very thick straightedge.

You'll get much cleaner folds if you pre-emboss the fold lines.

I did my usual embossing before the fins flats were cut off the full sheet. Trust me, it's easier to emboss when the fins are still on the sheet.

I used the un-serrated, smooth thin tip of a butter knife. Set a straight edge down the line, allowing a little space for the thickness of the knife. 

TIP: If you emboss over corrugated cardboard, be sure the inside ribs of the cardboard run parallel to the lines you are embossing. 

This is the back, unprinted side of a fin.

Look close and you can see the embossed lines.

Don't just do a single, sharp fold - 

Work up to a sharp fold line. Coax the tab folds little by little and finish with a sharp crease with a burnisher.