There are numerous examples and tutorials online, some very intricate, but most of them use specialty hole punches and shapes. If you've done some scrapbooking you may have those, but I don't so I looked for cards with windows that had simple shapes (circles, rectangles and other basic shapes) that could be created by tracing or drawing items to make cutting out the window fairly easy. Here are some examples:
I've included two how-to videos, because they use different techniques for affixing the window backing. For the window I've seen people use overhead projector sheets, photo sheets or acetate sheets bought online or at craft stores. I suppose acetate from mailers could also be recycled if the sheet was big enough, but I haven't tried that.
Here was the simplest (and probably the shortest) how-to I found. You can, of course, embellish this.
That one used foam strips to affix the window and sequins for the shaker part. This one uses double-sided tape and beads, and she shows you how to add a flat bow/ribbon.
If you'd like something more intricate, there are many videos online, and often they provide a materials list. Also, if you're prefer to buy a shaker card, Etsy offers a good selection.