Two problems are common when learning to use a Boston shaker. The first is, how do I close it so that it doesn't leak? The second is, how do I open the darn thing to extract my yummy cocktail?
Both have simple, but non-intuitive solutions.
To close a shaker so that it doesn't leak when you're shaking, you need to ensure you have a tight seal. The best way to do that is to place the mixing glass right side up on your table, bar, or counter, with the liquid and ice inside the glass (not the tin). Upend the empty metal tin so that the bottom is pointing toward the ceiling or sky, and place the tin over the glass. Take the heel of your hand and smack it on the bottom of the tin. This should form a tight seal. You don't need to hit the tin hard, just firmly. If you're too aggressive, you can risk breaking the glass.
Test the seal by lifting the shaker up while holding only the tin. If you lift the tin and the glass stays on the table, you don't have a seal. If you lift the tin and the whole thing rises up, you're golden.
To break the seal when you're done shaking, hold the contraption in one hand, and look at it carefully. What you'll see is that the tin and glass are in a curved shape. On the inner part of the curve, the tin and glass are snuggled up together like two birds of a feather should be. On the outer part of the curve, there's a wider gap between them.
What you're looking for is the spot where the two start to separate, as in the picture below:
This is where you'll aim to hit the thing to break it apart. Again, you don't need to hit it hard, just firmly, right where the gap is starting to form, and again, you'll use the heel of your hand.