Choose a section of fencing that you will remove to replace with a gate. Measure the distance between the posts on either side of the section you selected at the top and bottom of the posts and the height of the posts above the ground. Purchase a gate that will match the material your fence is built from while fitting between the existing posts.
Remove the fencing from the section you picked so that the sides of the posts that face each other are free of fencing and fasteners. Measure the difference in height between the gate and the gate posts. Measure up from ground level using this measurement and mark the post that will support the gate.
Measure the height of the gate and divide it into equal lengths according to the number of strap hinges needed to support the gate's weight. Plan on installing at least two hinges. Install the strap hinges so they are spaced equally along the edge of the gate. Use the carpenter's level to position the straps and ensure they are horizontally level.
Hold the gate level between the gate posts so the hinge plates are flat against the side of the post. Stand on the side of the gate where the strap hinges are fastened to the gate, and push the gate away from you until the hinge plates reach the edge of the post. Level the gate and fasten the strap hinges to the gate post using screws driven into the post through the predrilled holes on the hinge plate.
Place the gate latch on the post opposite the strap hinges, at a height you can comfortably reach. Install the latch hardware on the gate and the post according to the manufacturer's instructions. Make sure the components attached to the gate and the post are horizontally level so the latch functions properly.