Pick your brambles on a dry day from a good country location preferably away from busy roads. Pick a few under ripe brambles, not the green but the red. The secret to all good jam making is to use only the best of the fruit that's good enough to eat but about 5% of red or under ripe brambles helps the setting of bramble jelly.
Put your berries into the jelly pan and cover well with cold water and briefly and carefully stir them with your hands to rinse them through and then pour off the surface water with any of the bits and pieces that float to the top. Put on to the heat and bring to the boil and simmer for as long as it takes to cook the fruit and release all the juices. Strain through a jelly bag and leave overnight if necessary till all the liquid is strained off. Depending on your jelly bag or straining methods it might be necessary to briefly re-boil the contents with some more water and repeat the process. You can see from the residue in the jelly bag if it is still rich in good bramble juices or if it is well strained and ready for the brown bin. The traditional recipe is to strain them once only and the resulting liquid will then be made into jelly by adding a pound of sugar for every pint of juice. In practice it's all down to quality of fruit and how little boiling off of water is necessary to reach a setting point. The key thing is to use as little water as possible to extract the juices but you will need, some say, about a pint of water for every kilo of brambles. I don't measure the water I just have enough in to just cover the brambles in the initial boiling and as little as possible in a second straining to get the juices flowing. Unfortunately there is no exact recipe as all fruits are different but what you are aiming for in all good jam making is to get the best from the fruit with the least amount of (boiling which reduces the flavour.) When you have your juices all extracted add a pound of sugar for every pint of juice and boil it up making sure the sugar is all dissolved before it reaches a rolling boil then simmer till you reach setting point which can be 105c on a sugar thermometer or by testing a teaspoonful on a cool saucer straight from the fridge. It just shows signs of wrinkling on the shiny surface when you push your finger through the cooling liquid. This is not a case of feeling it thicken or it resembling a setting jam or jelly it is quite subtle and an acquired skill. You're looking for the light on the shiny surface beginning to ripple away from your finger. If it is showing signs of setting into jelly when tested then you have boiled it far too long and it will set into a thick mess you will have to cut out of the jar with a knife and re-boil with more water to repair the damage and it will of course be inferior quality apart from the fact that it will have been a lot more work and effort and will probably put you off jelly making for life. When you are happy that you have reached the setting point pour it into pre-washed jam jars that have been put in the oven for 15mins at about 100c or a low heat and if using screw on lids put the screw lids on while it is still hot but safe to handle with a tea towel.