Use a tape measure to determine the width and height of your split-rail gate. That will affect your decisions on framing and bracing--the bigger the gate, the more support you will need. If your fence does not already have end posts for a gate, you need to set those. That involves digging a hole to a depth about a third the height of your fence posts and setting the posts in concrete. Make sure posts are straight, plumb and solid.
Build a rectangular frame to fit the dimensions of your gate opening, allowing an inch or two on either side for hinges and latches. Use larger rails for the upright sides--usually the same size as your gate posts--and add some cross rails. The number, size and placement of cross rails will depend on the size and style of your gate--at least two and possibly three or four. You can drill holes in your side posts to hold the cross members, or simply nail them in place with strong galvanized nails. Make sure your frame is square.
Install cross-bracing. Again, this is a matter of gate size and style. Some longer gates use diagonal bracing on opposing sides of the gate with triangular peaks opposite, one side up when the other is down. Some gates use an upright brace in the center, with diagonal braces on either side, from the gate bottom to the top of the center brace, nailed into the horizontal cross rails. This is an easy and attractive option; for a really wide gate, you can use two vertical braces with two sets of diagonal braces.
Add hinges and latch. Use galvanized or other weatherproofed hinges to prevent rust: at least two and preferably three for a taller gate. Get the strongest hinges you can buy--a split rail gate will be heavy--and mount them with big lag bolts which screw into the posts. Some hinges come with a center pin you can remove for hinge installation; others use a fixed pin which mounts to the outside post, with a circular element which slides over the pin. Latches come in several styles--choose one that will secure your gate against wind or bumps from people or animals.