|gumpaste lace applied and overpiped|
The quickest and easiest way, in my opinion, is to just pipe it on freehand or by using a pattern that you've traced onto the cake. This usually works better with a really small tip, nothing larger than a 2 at the most. Here's a description of the easiest way to do that: http://acaketorememberva.blogspot.com/2011/01/piping-lace-on-cakes.html
Another way to do it if you stink at piping, which is apparently the new trend, is to use lace molds. This is also a good way to get a lot of detail, probably more than you'd be able to if you piped the pattern on.
When you use a lace mold an important thing to remember is that you need to use a piece of whatever you're molding that's thick enough to impress a lot of detail, but thin enough to stick to the cake and not look like a big lump of glop. One way to do that is to start with a medium-thick piece of fondant or gumpaste and press it into the mold, then REALLY press it in, using a piece of plastic wrap or gloves to keep your hands from sticking to it and making it shift.
Some lace molds have a self-cutting edge, but I prefer to use an exacto knife to trim the edges. You can also use it to remove sections that would look good if the cake underneath showed through. The more sections like that, the better the lace look will be.
If you use a debossing method, which just means that you're imprinting the pattern into the icing or fondant, you'll need either an impression mat or a homemade press. Once you have that you basically just press the design into the side of the cake. This works best with a crusting buttercream...Fondant will work to a certain extent but it tends to look better on dummy cakes where you can really go to town pressing the design in. Meringue buttercreams are either going to be too soft or too hard, depending on temperature, to get a good, neat-looking design on them using a press.
You can use an actual piece of lace that has a good edge and stitching on it, like a Venetian lace, to impress a pattern into icing. This wouldn't work as well on fondant because the lace itself is going to be too soft.
Once the lace is applies to the cake, however you choose to do it, you can either leave it the way that it is or overpipe it to emphasize the pattern. The square cake at the top of this entry is overpiped around the edges of the pieces. Doing this just emphasizes the texture of the lace and gives it more of a three-dimensional feel.
You can get lace molds in my Etsy shop in the silicone molds section: http://www.etsy.com/shop/ACakeToRemember?section_id=7687858
They're also available through other online retailers, just do a google search for "lace molds" (or "lace moulds" if you want the British results).
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA