Step 1: What You Need
Your favourite entertainment equipment, which might include tv, radio, hifi, computer, reading light etc.
Poles - they could be the very stout bamboo poles or half to one inch diameter wood dowelling. or any small section wood you can scrounge from anything. You will need 4 poles for the uprights which will need to be about 5 feet long (1.6m) and 2 which are the length of the bed plus about 2 feet (0.6m) and 2 which are the width of the bed.
Any fabric - thick dark fabric is best. For a single bed you will probably need the following pieces: top = 4x6 ft, 2 ends of 4x5 ft, 1 side of 5x8 ft, 2 part sides of 4x5ft. see basic pattern in the picture.
Decorations of your choice - found items are ideal
Step 2: What You Do
Tie the 5-ft poles to the legs of the bed - zipties would be ideal. These will be the four uprights at the corners of the bed.
Lash the other poles to the tops of these uprights, such that the extra length of the longest poles extend at the foot of your bed.
Make the fabric into a ceiling and curtains to go all round the bed. The curtain at the side you get out of your bed should be in two halves so you can easily part it to get in and out of bed, but the rest can be a single run round the other 3 sides. Essentially you are making a gigantic fabric box to drop over the top of the framework you have just fixed to the bed. It is not necessary to have the fabric run along the poles at all.
Install your entertainment items at the foot of the bed, WITHIN but not touching the curtains. This is essential.
Decorate to your choice inside and out. My mum was an egyptologist so her bed tent looked like some bedouin encampment.
Step 3: How You Save Energy
This is particuarly good if you are living alone on a low fixed income as you basically retire to bed with a good book, CD, DVD, blog of your choice and the energy from the TV, light, computer or hifi heats up the inside of your tent. You will be amazed at how much heat these things produce. In fact you will need to be careful that the curtains around the foot of the bed are not touching all the kit, as we dont want a fire.
My mum did this for years and was going to go on the local TV until her pals at Age Concern pointed out that if the UK government got the idea that poor pensioners could keep warm this way, they would stop the annual cold weather energy payments to them.