From Chocolatiers Wiki
There are several different ways to temper chocolate. Most chips will come already properly tempered, especially from nicer brands, and careful melting will maintain the proper crystal structure. For old chocolate, chocolate from cheaper brands, or chocolate that you need to fully melt, you may want to temper it yourself, which will require melting it on the stovetop and using a candy thermometer.
For both stovetop methods, it is possible to heat directly on the stove, but requires very close attention and risks burning the chocolate. It's much easier to use a double-boiler.
Set aside several chips to use as seed crystals. Heat the chocolate until the chips have almost all melted. Pull off heat and stir until the remaining solid pieces melt. If all chips melt, break up the extra chips and add them in to "seed" the proper tempering back into the melted chocolate.
Set aside several chips to use as seed crystals. Put the remainder of the chocolate in a microwave-safe bowl, and heat at full power for 30 seconds. Stir the chocolate for 30 seconds. Repeat heating and stirring until the chocolate is almost melted. You should try to get the chocolate such that after the last round of heating, there are still chips but upon stirring the rest of the chips just barely melt. This may require shorter heating for the last round.