I am not a concrete specialist, but my DIY approach has been as follows:
Stretch a string line to set grade and alignment. If sidewalk is adjacent to your house, grade away from the house. Nail forms conforming to the string line using 2x12 stakes 4 feet apart. You should drive the stakes, then nail the forms and cut off the stakes so you can drag a screed over the top without hitting the stakes. Dig out your 6" dept and be sure to add enough width for forms and stakes. You don't need to dig the entire depth because you can use fill (top soil) to grade away from the finished walk. This can save a lot of digging. Dig out high areas and fill low areas with dirt, sand or gravel, then compact grade. 2" Course gravel underlay is recommended. Most concrete requires metal reinforcement or fiber mix. Check with your concrete supplier to get recommendations or to see if reinforcement is included with your contract. Fiber reinforcement added to the concrete will increase the per-yard cost a little, but is much easier than laying rebar.
Install expansion joints against existing concrete, driveways, patios, entries, etc.). This will allow your concrete to expand, due to high temperature in the summer months. One square yard of concrete will spread 81 square feet at a 4 in. depth.