How you extend your front porch concrete will depend on the existing porch, the new area you want to cover and how you want to finish the extension. Always check local building regulations first -- many areas have restrictions on street setbacks and other conditions that will affect a porch extension. Your first decision will be height. If your existing porch is just a slab, an extension will be easy. If the porch is elevated, extending it will be a more complex undertaking.
Measure the area where you want to extend your front porch concrete and mark it with wooden stakes and builder's twine. If it will be a slab extension, you can stake just the desired perimeter. If it will be elevated, you will need to add 6 inches on all sides to permit pouring of footings for a perimeter wall. Then excavate the area with a shovel. For a slab extension, dig about 10 inches to allow for a six-inch gravel base and four-inch slab. For an elevated porch, dig a perimeter trench at least eight inches deep and eight inches wide, then excavate the interior four to six inches, depending on how high your finished extension will be.
Compact the soil with a hand or mechanical tamper, then frame your concrete extension area. For a slab, stake 2-inch by 4-inch boards around the perimeter. Make sure all corners are square, then nail the corners together. For a higher porch, pour concrete footings in the perimeter trench, let the concrete sit at least a day, then build forms for a wall four to six inches below where you want your finished surface. Make walls on both sides and stake them in place securely. Be sure the tops of the wall forms are level all the way around.
Build a gravel base. Fill the interior with medium gravel and compact it thoroughly. It's best to pour it in layers, compacting each layer before adding another. For a slab, build the gravel base up to the bottom of your 2-inch by 4-inch forms. For a higher extension, make the base to four to six inches below where you want your finished surface, depending on how thick you want your concrete top to be. Then add an expansion barrier to the existing front porch concrete. Use a rubberized or asphalt material, 3/8-inch to 1/2-inch thick. Fasten this material to the existing concrete with construction adhesive.
Place welded wire or other reinforcing over the gravel base and then pour your concrete. Mix concrete in a wheelbarrow for very small areas. Rent a mixer or hire a concrete company for larger areas. Pour the area full of concrete, then screed it by dragging a 2-inch by 4-inch board or similar instrument over the surface to compact and level the concrete. Once it has begun to set, use a trowel to smooth the surface. If you want a non-slip texture, sweep it lightly with a broom. Then use an edging tool to finish the edges. Very large surfaces will require grooved expansion joints every six feet.