Maybe it will help if I give you a step-by-step account of how I do it. I use
QCad for designing wooden slingshots which I cut from plywood or mahogony. If
the object you are designing has left and right (like my slingshots) or top and
bottom mirror images, it is only necessary to draw one side. I export to .png,
then massage that image with Gimp. I copy and flip the left side, then paste the
two sides together to get a perfectly symmetrical image.That image is then
trimmed so that the top of the forks, the bottom of the handle, and the two
widest points are on the borders of the image. When I import that image into
OO.o I can then scale it to whatever size I want by right-clicking the pasted
image and setting the properties. Select "Picture", check "Keep Ratio", then set
either the width or the height to the desired setting. I cut the printed image
out, paste it to 1/4 inch plywood and cut my patterns from there.