Really hard work, but absolutely fantastic. This method explains how to do everything from scratch. If you are not able to do this or do not want to, you can also buy good ready-made birch syrup which looks like it has been handmade.
It is important to tap the sap at the exact time when it starts to rise up the trunk of the birch tree. In the south this usually happens at the end of March, but in the north you may have to wait until May. It you are fortunate enough to find the right tree at the right time, you will be able to tap one liter of birch sap in just a few hours. You will need some sap-tapping equipment, including a tree saw, some plastic drinks bottles and some string. Birch sap is harvested fresh, so it will only keep in the fridge for one week. After that you will either need to make the syrup or freeze it.
1 liter of sap contains approx. 1 tbs. of sugar, so in order to obtain ¼ cup (0.5 dl) of syrup you will need to start with five to six liters of sap. You should check the sap as it starts to thicken while you are cooking it, because it can burn easily. The sap should be dark brown when it is ready.
Find a tree which still has mouse ears (leaves which are just starting to unfold). The easiest method involves sawing off a branch from the birch tree and tying a bottle which will hold 3 cups (0.75 l) of liquid to the branch so that it sticks down into the bottle. It is important that the branch should drop slightly downwards.
Collect the sap from the trees after a couple of days.
Sieve the sap and pour it into a pan which can hold 1½ gallons (6 liters) of liquid. Boil the sap over a medium heat until 1 per cent of the sap remains.