Drive two stakes into the ground, one at each corner of one end of the bridge, far enough back from the edge to be on firm ground and at least 24 inches apart. Tie mason twine to the stakes and also to the loose stakes for the other side of the bridge. Toss the loose stakes across the gap and stretch the line taut. Drive the stakes into the ground on the opposite side of the chasm, equally spaced with the first two stakes. Hang a pair of string levels, one on each piece of line.
Move the strings up or down until the bubble in the level is centered in the indicator and the two lines are at the same height, at least 12 inches above the ground. Measure the space between the stakes on either side of your proposed bridge. Using a circular saw, cut a piece of 4 by 6 treated lumber to the measured length for each side.
Dig a post hole just inside of the stakes on either side. Dig the holes 12 inches across and 36 inches deep as measured from the mason twine. Using the circular saw, cut a piece of 12-inch cardboard concrete tube form, 36 inches long for each hole. Set the forms into the holes.
Mix enough concrete to fill each form. Fill the forms and insert a 12-inch long piece of 5/8 allthread 7 inches deep into the concrete. Use a string level to make sure the pieces of allthread are upright. Allow the concrete to set overnight. These are your corner piers.
Measure and mark two 4 by 6 planks for drilling pilot holes, so that the 4 by 6 will fit over the allthread. Position the holes to correspond to the four corner pier (allthread) bolts you just positioned. Drill one pilot hole in each end of each of the two 4 by 6 planks with a 5/8-inch drill bit. With a 1-inch drill bit, drill into one face of each plank in each corner, to a depth of 1 inch; the nut will fit into this countersunk, 1-inch portion.
Set the 4 by 6 planks onto the corner piers with the holes you just bored over the bolts with the face containing the 1-inch diameter countersinks, facing up. Thread a washer and nut onto each bolt and tighten the nuts with a socket wrench, so that they fit snugly against the beam, in the bottom of the 1-inch countersunk portion of the pilot hole.
Using a miter saw, cut enough treated 5/4 by 6-inch decking to fill the length of the bridge. Treated 5/4 by 6-inch decking is the standard board used for the flooring of outdoor decks, available wherever lumber is sold. Make them long enough to fit perpendicular to the 4 by 6 side beams, with the ends of the decking reaching from outside to outside of the 4 by 6 pieces.
Attach the decking to the beams with a cordless drill and 1 5/8-inch treated deck screws. Use two screws in each end of every board.