Magnetic bag clasps are really handy for lots of projects, and they are super easy to fit once you know how. Just follow the steps below
Clasps have a male part
and a female part, (guess how they named those!)
and two backing or guard discs.
Normally in bag making, the male part would be attached to the underside of a flap on the bag lining, and the female part, which is bulkier would sit on the outer of the main body of the bag.
First, mark the placement for the male part of the magnetic clasp by adding a small dot on the fabric in chalk or disappearing pen. Make sure it is far enough from the edge of the fabric so you still have room to sew a seam as required afterwards. I like to fuse an extra little square of interlining on top here just for added strength, particularly if I am using a fine or open weave fabric.
Now lay the little metal disc (the guard) on top of this hole so it lines up with the dot showing through the centre, then mark the two slits at each side with your pen.
Remove the disc. You have 3 marks. We are just going to use the two outer lines.
Using small sharp scissors or a seam ripper, carefully snip through the 2 slit marks – just enough to allow the prongs to push through.
Now turn your lining piece over so the right side is on top. You should see the 2 slits you have made.
Take the two pieces of your clasp and choose the thinner male clasp side (yes the one with the bit that sticks out) and push the prongs through the slits.
Turn the fabric over again to the wrong side and place the guard disc over the prongs, now bend the prongs outwards to secure the clasp. (You can bend the prongs inwards if you prefer, but I find it is less bulky in the finished bag when bent to the outside.) 
Time to add the magnetic clasp to the outer fabric
Follow the same steps to insert the female part of the clasp.
Mark your required position with a dot in chalk or disappearing pen on the reverse of the fabric. Add a square of interlining for strength. Place the metal disc over the dot again and mark the slit position. Carefully slice through the slit marks.
Turn your fabric over and insert the prongs of the female part of the clasps through the slits
Add your guard disc to the reverse side and fold prongs back to secure.
You can buy good quality magnetic clasps cheaply from my shop here
There, simple once you know how – now go forth and add clasps to everything without fear !