The basic principle behind flashing is to add a small amount of exposure to the print. Small increments affect highlights more than shadows, so that small increment tends to reduce contrast. Its a very effective way to add just a hint of detail in highlights that would otherwise be blown out. I tend to use post-flashing rather than pre-flashing simply because for me its part of the process of creating the print.
The textbooks tell you to either remove the negative from the carrier, and then give a short burst of light using the enlarger, or else use a secondary light source. There are some textbooks that talk about using a small flashlight that is masked with a bit of neutral density filter (which could be nothing more than a bit of exposed and processed film leader) that you can use to flash specific highlight areas. I've tried that - it does work, but it's tricky to get the right amount of contrast reduction and avoid creating an obvious abnormal shadow.
The approach that I finally settled on and that works for me is to have a sheet of matte mylar drafting film (it's been around for a while!) mounted in a cardboard frame. The sheet of mylar is fairly large compared with the negatives that I normally print - about 8x10". If I conclude from an initial test print that the highlights need to be brought down a bit, I make a second print with all of the burning and dodging that I want to do. But before removing the paper from the easel, I hold the mylar sheet under the lens and make a final flashing exposure that is typically 5-20% of the base exposure. The mylar absorbs some of the light, hence the duration of the flash in my approach may be longer than in other methods. and the mylar also diffuses the light so that there is no image content. The main advantage of this approach is that it allows me to flash the print, but doesn't require that I tinker with the negative or enlarger settings in any way - so that it can be integrated into a printing sequence without affecting the time required to make the NEXT print.