A well fitting harness should be adjusted so that it is not really tight (you need to be able to get out of it easily) but not so loose that the set rolls around on your back which would be really uncomfortable. I’ve drafted out some instructions to help you get it right for you.
A diver's harness should be rigged from one piece of webbing and should have no quick-release buckles or other failure points. Though plastic quick-release buckles seem to simplify the process of getting into and out of one's dive gear, these “savings” are illusory, putting a diver at greater risk that s/he would be without it. In many cases this could prove fatal as the diver clings to tanks whose negative tendency stands in stark contrast to his/her own positive tendency. In cases where the loosened or dislodged quick release does not cause a diver to lose their tanks it can easily cause a dangerously significant shift in weight, throwing the diver off balance.
To get your harness to fit well, wear your dry suit and under suit just as you would when diving. I start with the shoulder straps. Put the harness on your shoulders without doing up the waist or crotch strap. Reach back and see if you can touch the top of the back plate – as in the picture to the right. A friend will be able to tell you if you are in the right area as sometimes we reach into the channel of the plate and think that we are missing it. The plate tends to end up roughly where a rear entry dry suit zip would be – perhaps a little higher depending on your reach.
When you are happy with this, take the plate off and check that the shoulder straps are of equal length as in the first photograph.
Next, do up the waist strap. The waist strap should go straight across from wherever the holes in the plate are on your body – don’t drag it down to where your waist really is – we are all different heights and some very tall people will find that it is over their chest rather than near their waist. For this reason, it shouldn't be too tight and it should never restrict your breathing.
Now the waist strap is in place, reach th rough your legs and bring up the crotch strap to check its length.
The crotch strap is also one piece, and has a loop in the front through which the waist belt is threaded. The crotch strap is needed to hold the diving system in place and to prevent it from sliding up toward the head when entering the water or when inverting; later it will also be necessary for any diving involving a diver propulsion vehicle (DPV). Once threaded through the crotch strap, the belt buckle should be placed to the right of center so as not to get accidentally opened by the crotch strap.2 The crotch strap is there to steady the rig, not to drag the waist down into the wrong position. A centimetre or so longer than necessary will allow easy access and removal of the strap in use.
To avoid the unnecessary clutter of multiple attachments, the DIR harness only supports one D-ring on each side of the chest, one D-ring on the diver's left hip and two D-rings on the crotch strap. The chest D-rings should be bent slightly so that only one hand is needed to clip bottles to them. The hip D-ring is used for pressure gauges, stage bottles, and other gear, while the crotch strap D-rings are used for DPV attachment (front) and towing (see above). Individuals should not clip equipment to the front D-ring, because it will hang too low and will create entanglements and drag; divers should clip off any additional gear that is being towed to the rear crotch strap D-ring.
The shoulder D-rings should be positioned relatively high on the shoulder straps, especially if you plan to utilize stage bottles in your diving. If the D-rings are too low, the stage bottles will hang too far away from your body, causing significant drag and possibly damaging marine environments.
The shoulder D-rings should also be high enough to allow for proper storage of your Scout back-up lights and to allow for ease of use of your wing’s inflator hose to adjust trim and buoyancy, yet not so high that you can’t clip an item off with the same side hand as the clip. Find out where the shoulder D rings should be by extending your arms when your eyes are closed. Bring in your thumbs and where they naturally fall is where the tri glides for the D rings should be.
The waist D-ring is used for clipping off the SPG and for stage bottles. To position the waist D-ring, run your left thumb straight down your left side until it hits the waist strap roughly in the centre of your body. Your thumb should intersect the D-ring, very close to your hip bone. If you habitually use a lot of stages and leashes this may have to be moved back slightly
The front crotch D-ring is only used with a tow-behind scooter. Gear should never be clipped to this D-ring, as it will hang in the slipstream and pose significant entanglement hazards. The back D ring should be worn where you can reach it taking into account the length of your tanks.
Remember, while standing on land, your harness should be snug but not uncomfortably tight. In the water your harness should be snug enough to prevent notable motion of the cylinder as you change orientation, but not too tight so as to bind or restrict your complete range of shoulder and arm movement.