I use two kettles set up as a double boiler over a LP gas burner, just hot enough to liquefy everything. Dip a 6" paint roller into melted wax, apply to frame fast and furious, need not be neat. The roller needs to be left in the melted wax while not rolling, else it will cool and not roll properly.
Just don't put too much that the bees can't see the hex pattern below it. If you put too much burr comb may be prevalent.
We also used our dark wax for this job. Once the bees draw it out it's hard to tell it was dark to begin with.