Look at your rebar placement plans and find the spacing required between each bar. This will be written as the type of bar "at" a particular measurement. For example, your plans may say "#4 bar at 12 inches." There may be different spacing required for each layer of bars, so pay close attention to the bar spacing designated on your plans. A line will be drawn from the spacing text to the bar it references (bars running to the north/south or east/west).
Determine which is the longer bar and which is the heaviest bar (the greater the bar number the heavier the bar) called for on your plan. If your heavier bar is heavier than the weight of your longest bar then this will be the first bar placed. If your rebar mat consists of two layers of the same weight bar then the longest bar will go down first.
Lay out the spacing for your first layer of rebar. Find the exact center of the area you will place the rebar in, divide the spacing in half and measure out from the center one half on each side. For example, if your spacing is 12 inches, measure out 6 inches from each side of the center point. Then, continue laying out marks (using spray paint) 12 inches apart until you reach the edge form for the area. If you have a very large area to place bars in, place layout marks every 20 feet so you can keep the bars straight. Place the reinforcing bars on your layout marks to form the first layer of the mat.
Get your rebar clearance right. Look at the placement plans for a note (usually in the upper right hand corner) about the required clearance distance between your first piece of rebar and the outside edge of your concrete. Typically it will be 3 inches. Go back to your spacing layout and adjust the position of the last bar so it is 3 inches away from the edge form. If this would place the bar greater then 12 inches away from the next, leave the bar in its original position and add an extra bar 3 inches away from the form.
Layout the spacing for the layer of rebar that crosses the one you placed to form the complete mat. Do this in the same manner as you did before in step 3 and 4 only place your spray paint marks on the actual bar you placed rather then the ground or the bottom of the form. This will make it easier to follow.
Tie the bars together wherever they cross using tie wire and linesman's pliers. Simply pass the wire under the crosspoint, cut the wire so you have two ends protruding about an inch above the upper bar. Twist the ends of the wire together by grabbing them with the pliers and spinning the pliers.
Lift the rebar mat onto concrete brick or steel support runners (whichever is specified in your plans). Make sure the mat is supported every 4 feet. Raising the mat will allow the concrete to flow underneath it and completely encase your reinforcing bars.