Level out the path your wooden walkway will follow. You don't need to get it level to engineering specifications, just flat enough that you can set down your 4-by-4 beams so they will not rock. Use your garden shovel and hoe. If the earth is particularly hard, consider wetting it first.
Measure the length and width of your garden path.
Buy enough 4-by-4 beams so that their total length is twice that of the length of your walkway.
Divide the length of your walkway by 4. This is the number of 2-by-4 beams you'll need, with each beam as long as your walkway is wide.
Lay two 4-by-4 beams parallel to each other, each on one side of the walkway area. For best results, set them at the beginning of the path.
Lay one 2-by-4 beam perpendicular to the 4-by-4 beams you just laid down. Its outside edge should be flush with the ends of the beams. Screw it in place using two wood screws at each point where the beams cross.
Lay a second 2-by-4 beam parallel to the first and half an inch away. Use your 1/2-inch piece of scrap lumber for spacing. Screw it in place as you did the first.
Repeat Step 3 to add more beams until you reach the end of your 4-by-4's.
Start again at Step 1 to build the second section of your walkway.