Magnetic snaps are fabulous! They are very affordable if you buy them online, and they are really easy to install.
But you can go beyond just cutting two slits and popping in the snap, and take a few extra steps to make them less likely to rip or wear through the fabric.
Here are some basics about the snaps, and the process I use to insert snaps on flaps.
1. The Parts.
Below are the parts needed to install a snap. For items with a flap or strap I always use the male part on the flap, strap, or whatever part you would pull to separate the snap pieces from one another.
2. Size and magnetism.
Since most the items I sell are on the smaller side I buy 14mm snaps as my all-purpose snaps. 18mm are also common and easy to find on etsy or ebay. I definitely recommend buying them online to get a good price. When I see them in sewing shops they are often priced at $2.00 - $3.00 for ONE snap.
Some of the snaps will come with an indication of the strength of the magnetic bond. Several years ago I bought the snaps below from JoAnn's and as you can see they are on the higher end of the magnetic scale. I could not even separate the snap parts from each other! The other snap that is not shown is pretty mangled from trying--ineffectually--to use pliers to separate the two pieces. Eventually I gave up. Look for snaps that are medium strength.
3. Lining and Interfacing.
If the fabric piece where the snap will be attached isn't interfaced I always use an extra layer of lining behind my regular lining. This serves two purposes--snap reinforcement and it also adds some extra weight and durability to the item.
For flaps I use another layer of thinner cotton on the opposite side from where the snap will be. This makes the snap prongs harder to feel (and see) through the fabric and it acts as a buffer layer for the fabric that will be in contact with the snap prongs. The prongs are often rough along the edges and I worry they might wear through the fabric.
In the picture above the snap would be attached through the green dot fabric and the next fabric layer. The light pink layer would be the buffer layer on the backside of the snap prongs.
4. Marking Snap Placement.
Use the snap bracket as a guideline to mark where to cut the slits for the prongs. This will ensure that you only cut them as big as needed to get the prongs through.
I recently started using Fray Check on the slits to seal the edges of the fabric and hopefully prevent ripping. Just dab a little of the sealant on the cuts and then insert your snap before the Fray Check dries.
5. Cotton Batting.
Finally I use a small square of cotton batting to back the snap and act as a buffer between the snap bracket and prongs and the layers of fabric it is attached to. You can use any scrap of fabric for this but I like the cotton batting because you the flatten prongs will sink into the batting making it harder to feel them on the finished item. It also acts as second reinforcement layer for the snap. Basically the more fabric behind the snap the less stress each layer of fabric will have and they'll be less like to rip.
Then simply flatten the prongs down using pliers.