See if the magazine is worth more with the ad or it is best to have the ad separately. It is possible that the magazine may be worth quite a bit on its own. If there are more than one ads that are in the collectibles category, you will definitely be better off removing them from the magazine.
Often these magazines are in a fragile condition as they have not been stored properly and may not have been printed in the best of papers to start off with. Make sure that when you are handling them, you exercise caution and do not try to force the issue as you may end up damaging the magazine.
If the magazine is stapled at the spine, remove these staples carefully and the magazine should open up for you to remove the ad. However, if it bound with glue, you will need to take extra care. First press the spine of the magazine down so that it is flat. Then very carefully pull apart its two halves away from each other. No matter how good the glue was back in the day, it is quite easy and even easier when the glue has been worn off with time.
Continue the process of separating the magazine until you have acquired the ad. You will probably need to use a paper cutter in order to remove it from the magazine as the last step.
The first thing after getting the magazine that you should do is to protect it. Put it in a plastic sleeve in its upright position and put it somewhere away from common access and light. The best way to save it is to get it framed.