First of all, finger press in about an inch (up to 3cm) of waste fabric at each end of the design. Make sure that the two ends are equal on the front (assuming that's the effect you want to create, of course!)
Using a sewing thread that matches your Aida band colour as closely as you can manage, make a few discreet stitches along the outside edge. Try to make them between the insides of the two edges so that the stitches don't show on the front. You can do this by catching the long stitches on the inner edge of the Aida band and joining them together.
There's no need to make any stitches along the straight edge and it's often better not to so as not to risk anything showing. Also, the edge of the fabric will almost certainly fray, so stitches are no real use here.
Still using the thread that matches the Aida band (in case you slip up and sew through, although you could use one that matches the felt instead, if you preferred), sew the two pieces together, again taking care to catch the long stitches on the band so that they don't show on the front. And here's the (back of) the finished item:
I hope that's been useful! If you use this tute (or part of it), please leave a link in the comments section to where we can admire a photo of your lovely new bookmark.=)
Text and images © Elizabeth Braun 2013