you can make it well in advance, a week or more depending on the lace. If you have sugarveil it does not keep as well as sugar dress (which is the best one ever to use..believe me I have tried most of them!) and crystal lace stays pretty flexible for a while also. I need to try cake lace so I cannot give an opinion about that. One trick with lace is to make sure you spread it well, and scrape it well, leave it about an hour and apply a second coat of lace to it. It will take generally longer than 3 hours to dry, when it is dry and not tacky to the touch, gently pry the mat away from the lace using a toothpick or something like that. then gently peel away mat once you have got the edges loose. I always have lace made about a week before I need to use it. good luck and if you have any questions feel free to ask.
Claire North
