I do a lot of cakes that have piped lace on them, and it's usually something that's based on the bride's dress.
If you have a piece of the actual lace it's a lot easier, but you usually don't. If the bride doesn't have really good photos of her dress lace, or she just wants a basic lace pattern, you can generally use details from a basic alencon lace and it will work fine.
The smaller the piping tip the better. I had to use a #2 round tip for the last lace cake that I did because I couldn't find my #1, and I thought that it was okay, but it could have been smaller.
First, figure out what sections of the lace you're going to be using. Lace generally has repeats in it, so if you choose one main section you can repeat that and give the impression of the entire pattern. You just want to be careful to make sure the pattern is the same size all over the cake, so you'll need to imprint the pattern into the icing in a uniform way.
There are cookie cutters that look like lace sections that you can use to mark off pieces on the cake to overpipe. Some of the ones from Lindy Smith would work well, and many simple flower cutters can be used to place the main flowers on the pattern.
There are also more complicated lace cutters that cane be used to press the pattern into the icing. Using something like this one will give you a good outline to follow.
If you want to freehand it you can do that, too, but it's a good idea to mark off the main sections before putting in the details freehanded. That way you'll have a basic structure to the pattern, and you can embellish the "framework" without having to worry about making sure the pattern lines up and meets at the right place.
You can also use one of those 3D pens to make a piece of the lace that you would then press into the cake to mark the pattern on the icing. I have one of these and it's so much fun...But if you don't have one the cutters work fine!
Here's a video showing some of the options to mark off the lace: