Anything more involved than line art usually involves some degree of beveling. Beveling uses an angled stamp to raise an element of the tooling above whatever's around it. Changing the depth and angle of the bevel can alter the prominence of that element as well and with all sorts of different stamps giving different effects beveling can be pretty darn versatile. It can be pretty tedious if you have a large area to bevel but when done well it makes a pretty profound and subtle effect.
In this case I was using two of my higher quality bevels. They aren't as robust as some of my other bevels but they make clear and smooth impressions and aren't as prone towards "chop". For each bookmark I'd use the longer bevel along any flat edges to make sure they were nice and straight. Then I'd go back over the interior of the letters with the smaller beveler pressing down by hand to flatten out the interior of the letter and bevel the curves.