You will likely need some sort of brewing software, I use the freebie version on beertools. From there it is a matter of adding 2-4 lbs (depending on your brewing equipment capacity) of base grain and removing the extract so that the OG remains the same.
At least that is essentially what I do. If it isn't the proper way, it has worked for me the last dozen or so batches.
Thanks for the replys, I'm new at this. This will be my first PM and once I get the equipment, I will go to AG. So it will help me with the amount of grain I need while subtracting the appropriate amount of DME? Or do I do that manually and just keep an eye on the OG? Also, if I am using light DME for an IPA, will it be able to help me pick my base grains to keep in that style?