The trick is to warm the custard up to body temperature and put it in just before they're going to wear them.They're less likely to feel it until they're past the point of no withdraw.
(
Mu Dinofiddler,
Thu 6 Aug 2009, 23:14,
archived)
On second glance through that also appears to be good instructions for use of rohypnol too. Mu Dinofiddler,
Thu 6 Aug 2009, 23:15,
archived)