First, The Inrun- make sure your inrun is acceptable, make sure there is enough speed, generally you will need more speed than you think, MAKE SURE that there will be little to no compression between inrun and transition into the jump.
Second, The jump- First measure out the base with your skis, a medium park jump is about 4-5 ski lengths long and 2-3 wide, you can obviously make it as wide as you want. Dig down until you can cut some blocks of snow in an area near where the jump is but not in the base area. Cut squarish blocks so that they can be stacked as straight as possible, stack the blocks back and two side walls around the perimeter until the jump takes shape. As you construct the outer walls fill in the center with the smaller garbage blocks that are left over. Once the jump is all filled in put on a pair of skis and pack it carefully paying attention to a continuous transition. Step out the inrun nice and wide.
Last, let it sit before hitting, For best results let it sit overnight