Some practice may be needed, if you have some severely damaged valentines or postcards to practice on that is good. These erasers are beveled perfectly to "shave" a thin layer of paper off. The idea when removing paste or backing is to moisten the backing and the paste enough to allow easy removal and to limit the amount of dampness which reaches the card, then to remove the paste/residue/backing while leaving the card surface intact. To get the dampness just right you might use cotton swabs then give the moisture a few minutes to work, then try it. You will find that all of a sudden the conditions are just right and the paste/residue will soften and easily scrape off leaving the card in good shape with only minor discoloration. Now most color printed postcards and valentines are not color fast so I would lay that card on a legal pad face down and avoid sliding the card around while removing the residue. Most important don't get in a hurry.
The truth about most of these Victorian valentines through the early 20th century is that the actual market value doesn't even come close to the value necessary to hire a professional conservator to archivally preserve (and probably never will). That doesn't mean that personal value or sentimental value doesn't justify some attention and expense to preserve for future generations.
I am of the opinion that old scrap book page material is far more damaging over the long run than carefully removing it. The quality of that old scrap book paper was really bad...high acid, high pulp paper. I put my postcards in acid free sleeves. If I leave that old high acid paper on the card it completely negates the use of the archival supplies. I have used this technique on cards valued at less than a dollar and some cards which would fetch several hundred dollars with good results.
Again we would all rather get mint condition without damage, we have no control over the past so we make the best of it. This card is a good example of just making me want to cry on one hand but being so thrilled with the image that I just have to 'grin and bear it'. This postcard was cut down by someone to make it fit into an album and had album backing on the card when I received it. I don't have a before but this is the after. The album paper was old pulpy black album paper. The paper was attached with paste in a few spots. The arrows indicate the discoloration which remains after the paper and paste removal. The corner damage was on the card when I received it. The damage on this card reduced the value by more than 1/2, it is still a very desirable and collectible image. I believe the card would sell higher in it's present condition than with old album paper and paste stuck to the back, and the acid damage has been stopped.