This involves using arranged materials to simulate a natural appearance, so enabling you to set up a comfortable and concealed shooting base within range of your targets, without arousing their suspicions.
The first factor in hide building is location. If the hide is 50 yards from the field where your quarry lurk, you aren't going to have much success. Observation is the key to a successful placement- remember that wind and weather may have an effect on where your quarry will be, and if the wind changes you don't want to send your scent to them. Prowl the area in full camouflage, but only take binoculars- watch where game congregates, then search for a suitable site there. The ideal hide has a hunting field of less that 180°, as it is tiring and awkward to turn yourself and the rifle to check if anything is behind you (not just to check for opportunities, but to check if anything is creeping up on you, it may raise a racket that will warn your intended quarry).
The best start to a hide is a natural hide, e.g. a fallen oak tree. A few pieces of suitable vegetation (preferably natural to the area) can then be woven in to cover any obvious gaps, but leaving sufficient space for a good field of fire. (Thistles are good for this- they don't take much careful arrangement, but a decent thickness will give you good camouflage). Failing this, a piece of camouflage netting is another good base. Size depends entirely on the area that you need to cover, but too large is always better than too small- you can tuck the edges under tree limbs or peg them down, making a better hide anyway. The better types of cam netting have scrim sown to the panels. Scrim is strips or leaf shapes of cammo material, and when out hunting, they blow in the wind and looks like leaves, branches etc. However, before using a scrim net, it is wise to blend the colours and get rid of any "shop" smell. To do this, leave the net out in the vegetable patch (or any other suitable patch of moist, muddy ground) for a month or 2, trampling and turning it occasionally. The colours will blend and flow, and the harsh outlines of the net panels will be softened by mud patches etc.
Two of the most important points in hide building are backlighting and removal of any hard, unnatural lines. Backlighting is one of the most important factors when hunting any creature with good eyesight (e.g. crows). No matter how good your hide, there have to be gaps in it to put your rifle through, and if the sun shines through these from the back of the hide (usually less well built), then any movements you make are as plain as though projected onto a white screen. Make sure you cover the back of the hide too, especially if hunting with the sun behind you. The hard shape of your rifle can be softened with strips of excess cammo material, or even loose bits of scrim. (Just a couple of panels around butt and barrel, and leave some bits dangling). Another use for any left over scrim is to fix it around your scope front lens with an elastic band to dull the glint when the sun catches it. If you use black stocking material, you can actually fix a couple of thicknesses over the front lens, and still see sufficient of your quarry. (Make sure you can still shoot accurately after any modifications of your kit, no matter how small- you don't want to find that you actually put the scrim too far over the lens and can't actually see anything out in the field!)