You can start by choosing a theme, adding a design, then customizing the text and colors that'll appear on the filter. Once you're done you can click "finish" to round it all out and finish your purchase. It's a much more streamlined process than before, and should be a lot simpler to create for birthdays, holidays and other various events you want to spruce up.
In addition to making Geofilters simpler to design and expanding the offerings, Snapchat has also rolled out an app update with a bundle of new features. New caption options including highlighting, bolding, italicizing and underlining text have made their appearance (finally) and you can now animate larger captions on top of your Snaps. Tapping on your face once lets you use Lenses now, and you can preview public stories via the Stories page before you add users as friends.
If you're interested in creating your own Geofilters, you can do so here now.