My chair hangs just a short way above the ground, making it easy to get in and out of and allowing my feet to rest comfortably on the ground when I am in it. You could hang the chair higher up but then you might need a ladder to get into it, which could be a hazardous exercise. Some commercially available hanging chairs have a foot rest, which would be easy enough to add if you want to be well above the ground and don't feel comfortable with your legs just dangling. You will spin and swing around a bit in the wind if your feet don't touch the ground, but that is part of the fun.
You will need
1.4 m (1 5/8 yd) of canvas (or other strong, rip-resistant, hardwearing fabric) that is at least 1.3 m (51") wide
Strong sewing thread
A reasonably powerful sewing machine fitted with a jeans needle and ideally a walking foot
2.7 m (3 yd) of 25 mm (1") wide webbing
A crayon, chalk or a biro
Ruler or tape measure
Strong scissors capable of cutting the canvas and webbing
One 28 mm (1 1/8") diameter wooden broom handle, at least 1.2 m (48") long
Two 23 mm (7/8") diameter wooden broom handles, at least 1 m (39") long
10 m (11 yd) of 7 mm (1/4") rope from a climbing shop (check the breaking strain is well above your weight)
A length of thicker rope - length depends on how high up your tree branch is
A hand drill or power drill with 8mm and 10mm wood bits
Teak oil or wood preservative
A carabiner capable of taking your weight plus the chair's (optional)
A suitable tree
Unless you live somewhere where the climate is dry, I suggest using canvas, webbing and sewing thread made from manmade fibres that will not rot if they get wet, because sooner or later your chair will get left out in the rain. Opt for polyester or nylon instead of cotton if you have the choice. Ideally, the canvas should have an open weave to stop rain collecting in it, but strength is more important than that.
The strength of this chair depends on the materials used and the quality of your stitching and knot-tying. Mine takes an adult male weighing 11.5 stone (160 lb, 73 kg) with ease, but you may need to beef up the components if the user is substantially heavier. When buying the rope and carabiner, check their load ratings. Carabiner-type keyrings are NOT suitable. Don't take chances, particularly if you want to use the chair to hang high above the ground. Make sure the branch you hang it from is sound wood and thick enough to bear the weight of the occupant. Remember that the load will be increased if the chair is used like a swing. Check the stitching and knots every now and again, and certainly before using the chair after it has been put away for a while.
I have given suggestions for the knots to be used, but the best knot for a job depends on such things as the flexibility and slipperiness of the rope, its fibre content, construction and diameter as well as what needs to be connected to what. If in doubt, consult a good book of knots or someone who understands such things - amateur sailors are usually good on knots.