Rhonda

You can add borders on all 4 sides and trim to the size you want

Or you could use 2 borders. It looks harder than it is because you sew to the top and have that top to hold while you cut the border.
I cut the first border to ½” before I add the next border. Then I add the second border and trim it also to ½” but if that intimidates you- you can cut it to 5/8” or ¾’. Remember the size it ends up is up to you. Your bookmark does not have to be any particular size. Whatever size it comes out as is just fine.

Here I decided to do my borders slightly differant. This is one of my favorite looks. I sewed the two borders together and then trimmed the dk one to 1/2". Then I sewed the dk border to the bookmark top. Repeat for the other side. Then trim the ends and add the double borders to the ends. trim. Iron all seams open for less bulk.

Tip: I trimmed the light green after I added the borders.