SOS recommends that users create labels, legends, and text in the form of
“PIPs,” (picture in a picture) which are images that can be projected on the
sphere. This makes it possible to avoid the warping that would accompany text
placed directly on a map, since the closer to the poles the text is, the more
it is warped when it is projected onto the sphere.
QGIS allows you to create PIPs using the print composer. This tutorial is a
good resource for learning to create most common map
accessories.
Once you have a legend that you like, you should save it as a separate image.
You can accomplish this by unchecking all of your map layers so that none of
them are visible in the print composer (note that you have to have them visible
when you open the composer, or the legend won’t be created in the first place),
and then exporting the image as a PNG. You may want to crop it down in an image
editor such as GIMP or a Microsoft Office product like Word or PowerPoint.
For example: if I wanted to create a legend for my earthquake data, I would
first open the print composer with one or more earthquake layers visible and
create a legend as described in the tutorial that was linked to above. Then I
would uncheck all the layers and click Update under the
Item Properties tab to ensure that the legend is the
only thing visible in the print composer, and click Export as
Image.