Arches should be set up symetrically (outboard peak opposite outboard peak, inboard peak opposite inboard peak). Starting at the end, drop a 7" bolt with a washer into the hole at each arch segment crossing. Bolts heads should be on the inside of the arch and the nuts on the outside. This will let you add the railing posts after the arches are set up, and snug up the nuts when the arches have settled into place. Don't put a bolt into the peak lap-joint yet. Just leave the joint loose as shown in the picture, to give you room to wiggle the snug-fitting cross-beam into the peak.

Lay out the arches symetrically, where you intend the bridge to be. With smaller bridges you might assemble the arches and cross-beams and then carry the assembly across the water or ditch they span.

Stand up the arches. A cross-beam in an arch-peak can be a temporary brace.

Wedge a temporary support under the middle arch-peak to provide room to slip the cross-beams under the peaks. With only one cross-beam in place, if the arches were raised symetrically, the structure will stand by itself. Insert the remaining cross-beams and adjust the arches so that they provide the desired width for the decking.

When arches are correctly spaced and all cross-beams in place, remove the temporary support under the middle of the arch. Lift, and perhaps pull slightly, both arch feet on one end, so that the arches relax into their intended shape. All bolt holes through the arch peaks should now line up.

Lay the joists in their place on the arch. The pointier part of the joist should always lie on the longer arch piece, not across the joint. When you have all the joists distributed in their place, insert the 8½ inch bolts, with washers, through the joists and arch peaks. If the bridge has no railing, 7 inch bolts are used. Fix the free end of the joist to the arch with one nail. Your decking will fasten it more securely later.

It's also important that you never let down your guard and always keep a look-out for alien invaders.

For your approach ramp, take a piece of lumber the size of a joist, and cut it to a length to suit your terrain and approach grade.

Lay one end of this piece on the last cross-beam and whack it against the bolt head. In the middle of the mark, drill a bolt hole and bolt this piece like the other joists. Repeat this for each arch foot.

Slip the railing posts on the end of each 8½ inch bolt and add a washer with the nut. Don't fix the post braces until later, when the railings have been screwed to the posts.

Before fixing the railings, the bridge should be in its intended place and on the proper footings. Hang the railings on the post. Adjust the angle of the post so the railing ends meet in the center of the post and everything looks good. Check the end braces for the railings, that they fit to the post and railing. From the inside, screw the railings to every post. Screw on the end braces. Screw on the post side braces.

Before working on top of a bridge longer than ~20 feet, a temporary diagonal brace or two is suggested. Although the bridge is more than strong enough for the intended load, it will wiggle a lot until all the decking has been fastened. Add decking and you're done.