How do you do it?

I could never keep the pencil straight! Then, aha. I saw an Incra jig for drawing straight layout lines with tiny holes for a pencil lead. Thus was born my arc/circle maker. An 18” lath (could be any length) with a series of 3/32 holes 1/4” apart and one for a finish nail in the end. Then, I went further and added a square of 1/2” ply to slip under the nail hole and used double stick tape to hold it to the work, then nailed into that.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

Draw it in CAD. Print it full size. Double-sided tape it to the work-piece. Cut to the line. Simples!

-- Martyn -- Boxologist, Pattern Juggler and Candyman of the visually challenging. http://www.theartofboxes.com

I have a box full of cartographer/drafter thick plastic curves. They work pretty well.

-- "Checking for square? what madness is this! The cabinet is square because I will it to be so!" Jeremy Greiner LJ Topic#20953 2011 Feb 2

Well, a formula for the radius and a long sticked jig to guide the router is my way for long arcs.

-- The smell of wood, coffee in the cup, the wife let's me do my thing, the lake is peaceful. http://gagnerwebsite.com/Deceiver/Craftsman_on_the_lake/Craftsman_on_the_lake.html