The service will locate any buried power lines, pipes, and other underground utilities. It is vital to wait for this before starting any digging, to avoid serious injury or property damage. In the U.S. and Canada, you can call the toll-free "Digline" number at 811. Knowing the depth, width, and length of your trench will help you save effort and keep your trench the shape you  desire. It may help you to mark widths, lengths and route of your trench using stakes and string. If available, you can use sandbags or other markers to outline the path of your trench.  If you are using the trench to install or replace electrical utilities or pipelines, you will want to dig at least 2.5 feet deep in order to protect the pipes from frost, but no deeper than 4 feet. The width of your trench will depend on your pipes, but will likely be narrow. If you are digging a trench for a sprinkler system, you may only need to dig 9-12 inches deep depending on the height of the sprinkler, and 5 inches wide depending, again, depending on your sprinkler system. Consult the instructions that came with your sprinkler system before installation. You’ll need a D-handle sharp shooter shovel and a trenching or clean-out shovel. These can be purchased at any hardware or gardening store. For clearing roots, pruning shear or a Pulaski digging tool can help you quickly remove this obstruction. Wearing gloves will protect your hand from blisters and splinters, and comfortable work boots will provide foot protection and traction. Be careful when you dig around trees or other pipes. Roots can add a significant time to your excavation, and a ruptured gas line will require you to call your gas provider immediately. Severed utility lines could also leave you without power until your electric company can fix the problem.  If you're digging near trees, make sure that your trench will not invade the tree’s protected root zone (usually the part of the roots that lie directly below its branches). If you're digging near pipes, try to determine where any other pipes may be. New pipes should be at least 1.5 feet away from another. Use the D-handle shovel to loosen the dirt along both sides of the soon-to-be trench. This will make it easier to dig out the dirt in the middle while physically establishing your dig line alongside the guideline you laid. Cut both sides of the hole with your shovel, break up the topsoil, and then work both sides of your trench until you have loosened enough soil to merit clearing. Once you have accumulated enough loose soil, use the trenching shovel to remove it out of your way. This may be a pile off to the side, or it may be a location you choose prior specifically for backfill. Depending on the depth and length of your trench, this could take a considerable amount of time and effort. Use your D-handle shovel to break up the soil and the trench shovel to clean it out until your trench is the desired length and depth.  Running into roots might require you to place the pointed end of your shovel on the root and stomp, which should sever most small-medium sized roots. More developed root systems might require a Pulaski digging tool. Pruning shears are another good option, if your shovel fails and you do not have a Pulaski digging tool on hand. An unsupported trench can be extremely dangerous, as collapsing soil can kill someone standing in the trench. Any trench 3 feet (0.91 m) (0.9m) deep, and some shallower trenches in soft soil, should be supported by side walls (such as timber posts and panels) before digging any deeper. You can increase safety by "benching" (digging in tiered levels), or by digging the walls at a slope instead of vertically. An experienced trench excavator may choose to keep the trench unsupported at depths up to 5 ft (1.5 m), but only under stable soil conditions. Follow the 3 ft (0.9 m) rule if you do not have expert supervision.

Summary:
Call a utility location service. Determine the trench dimensions and purpose. Purchase supplies. Avoid obstructions. Break up the dirt. Dig out the trench middle. Continue loosening and clearing the soil. Take safety measures with deep trenches.