There are several ways depending on what is required. Of course there is negative masking, but there are a few other more simple techniques I sometimes use (both require a second light source/enlarger and easel - I bought a small used Durst for $10 for this purpose so I can use filters etc):
1. With the flashing easel set to the same size as the print, you can tape down little markers, pointers etc so you know where to position the burning cards/holes etc during the flash exposure.
2. Make an actual scrap print, cut out the areas to be flashed, and place the print on or above the paper during the flash exposure (essentially paper plane masking). Note it is best to use RC paper for this since in addition to it being obviously easier to work with and faster to process/wash/dry, it has to be dimensionally stable paper so that "registration" at the paper plane works properly.
3. Combine (1) and (2), burning/dodging during the flash exposure with a print that has cutouts etc.
Diffusing paper can be used with any of these techniques to feather edges etc.
I originally learnt about localized flashing from John Sexton's masterful use of it in many of the powerplant, Hoover Dam and space shuttle prints in his "Places of Power" series. These prints remain my reference point for how to manage extreme contrast situations without destroying local contrast.