The easiest way is to drill two holes through your bumper and put as wide and long of a piece of metal as you can fit under the plastic to distribute the load. You can weld flat metal bar to a stout piece of the front end and run it through the grille or bumper by cutting slots out too. Remember to reinforce more than you think as bumping around will wear out your support, especially the farther away from the attachment point.
The hardest thing will be to keep them from vibrating which is really annoying. If I were doing it (and didn't care about resale value...much) I'd probably suggest cutting out the rubber bumper cover and attaching some flat bar stock to the underside of the bumper beam. Getting rid of the rubber/plastic in between the pieces of steel will probably solve the vibration issue. The downside is the lights will be horrifically exposed in parking situations.
If you search 'light bar' on here you should find several pictures of alternatives that are more rally-esque (or just use Google images).