... Oh, and a Silver Birch tree.
First find a Birch twig, scrape the bark off, trim it to a point at one end and a flat at the other.
Drive your knife in at 45% and make a clean cut about 1.5cm deep. Lift the blade ever so slightly to open the flap of bark but not so much as to tear the fibres.
Immediately there should be a trickle of sap. Place the flat of your prepared stick into the slot, and encourage the sap to run down its length.
Tie your cup to the tree and wait. It will take some time to fill depending on the tree and conditions. A large specimen near water will produce a lot of sap but is never quite as sweet.
There always seems to be a trickle that escapes down the tree, and a second cup placed below a natural drip point can save waste. Bear in mind that if your are somewhere where animals might have urinated on the tree, then this is not a good idea!
When your cup is full then pull out the stick and firmly press the flap of bark shut and hold it for a minute or two. You won't stop the flow entirely, but sufficiently to allow the tree to heal itself.
You can drink the sap fresh (the earlier in the season, the sweeter it is), use it for tea, drinks, or carefully heat it in a pan (don't boil it) to reduce to a syrup. It'll keep for a couple of days in the fridge, and is freezable, but is best drunk straight from your collecting cup... Cheers!