A while ago I wrote an article on how you can easily create bokeh with just about any lens. See: What is Bokeh and how do I photograph it? In this article I would like to explain very simple, and incredibly cheap, ways of modifying the bokeh you get into different shapes.
There are a number of attachments you can purchase which will modify the bokeh for you, but if you are on a budget or rarely use this method you can use various self made “filters” which do the same thing. Paper is the simplest and most inexpensive way to create this effect and experiment with until you get the look you want. Using thinner white paper will produce white vignetting and using black will produce darker vignetting. You could even experiment with various colours of paper!
Take a square of paper with enough space on the sides so you can use an elastic band to tie it around the front of your lens. Cut a shape into the paper (NOT while attached to the lens!) and then use the elastic band to tie around the head of the lens as I did in the image below.
In my case I was not too concerned with the shape of the cutout, but if the shape is important to you then make sure the paper is parallel to the lens surface. You can experiment with various sizes of the shape, but the larger it is the harder it is to get the shape to show up. It doesn’t even need to be in the center, but bear in mind that if it is not centered then your focus point needs to shift with it.
The paper I stuck on my lens was slanted backwards (the top was closer to the lens than the bottom) and here was my result (I used a Minolta 50mm lens at f/4).
If you want to make your bokeh shapes re-useable you could cut them into hard card paper then use a spare lens cap with a large circular hole cut in the middle (leave enough room at edges for stick down) as a mounting with some tape or prestik.
Have some fun with this!