Pretty easy: make a little making jig like a piece of rib and run a white line under the gunnel where you wish with a white marking pencil . I use the " prismacolor " one found at any art store. Keep it sharp and use a light touch, even marking only every foot for a few inches. The white should easily wipe off after the painting. I run a masking tape line down the line marks and then another 1/8 " below. You could run another line below the first easily if you were to use a stepped jig with both line points established under the gunnel with one pass, if you will. You now have two masking lines ready to run. I have used the Scotch masking tape but I prefer the very thin mylar product called " The Edge ". It, in the 1/8 and 1/16 " will bend around any curve without kinking and hardly leaves an edge...like .040" . You will appreciate this if you start adding much in the way of design to finish the canoe. Be careful when removing the tape and check for stray paint before you have the glass of wine. Oh, and use as thin a coat as you can to cover in one coat as it will want to move down to the top edge of the bottom line and show a not-so-pretty-wave along the whole line if you go too heavy.
Have fun, Dave