1. SAFETY FIRST FOR YOURSELF AND THE CUSTOMER!!
2. Always disconnect the power going to the unit you are repairing.
3. You should wear safety glasses when doing repairs.
4. Jewelry can become caught on the unit you are repairing and can cause you to get hurt or cause damage to the unit.  You should not wear any jewelry when performing repairs.
5. Always use good judgment on any repair.  Your safety is most important.
6. Never by pass any safety switches
7.Make sure all wiring is safely away from any moving parts of the unit.
8.While you are doing the repair you should check the unit for any electrical components or wiring that may be exposed, wet, burnt, or frayed.  Any of the conditions could create an electrical or fire hazard for you and the customer.
9.During the repair should you notice any conditions that are hazardous; electrical shock or fire hazard; ALWAYS CALL THIS TO THE CUSTOMER ATTENTION.  If the customer does not want you to perform the repair that would correct the situation; you MUST put a note of the condition and indicate it is dangerous on the receipt and have the customer sign next to where you wrote it. 
10. Never perform any temporary repairs that could in any way endanger the customer or appliance.
11. Always put the unit you worked on through a complete cycle before leaving the customers home.
You should be familiar with any electrical codes that may apply to your, state, region or locality, regarding any repair you will perform. 

The problems and solutions provided is to help you diagnose the problem it cannot pin point any given scenario but will eliminate the most common problems.  Your mechanical and electrical abilities will determine how you interpret this information and how you apply it.  There will be times when one or more condition exists at the same time.  The information provided is to give you a starting point for the most frequent/common problems.

For gas appliance repairs you should have a gas leak detector and a carbon monoxide (CO²) detector.  This equipment will pay for itself in a short time.  This is an additional charge to check the appliance out and is well worth it; for safety and efficiency.

For all appliance wiring, pick up an assortment of colored tape from a radio shack store or home improvement store.  I recommend blue, yellow, red, black, green and white. 

Draw a picture of the way the wires connect and color code them.  There will be times when you have 3 wires or 2 or 3 black wires of the same size.  This way, you will remember where they went and save you time tracing them if you forget which went to which.

When performing any repairs to any appliance always make sure no power is going to the unit.  When testing any appliance always make sure no power is going to unit; except when the test requires power to the unit-be extremely careful when performing any test with the power on- danger of severe injury, shock or death.

With any appliance that will not start always carefully check the power supply to the appliance first.  An electric dryer requires 220 volts to operate the heater and motor.  Washers, dryers, stand alone freezers, refrigerators, gas dryers require 110 volts. 

Before moving any electric dryer or electric range; look to see if it is hardwired. 

Some dryers, ranges and other appliances will be hard wired.  This means they are directly wired to the power source; no plug comes from the appliance into an outlet.  If this condition exists make sure there is a strain relief correctly installed on the appliance.   The strain relief prevents the power supply from moving or becoming disconnected; this creates a danger of severe injury or death.  It is absolutely imperative the strain relief is correctly installed on any appliance.  An improperly installed strain relief is also as dangerous a hazard as no strain relief.

I always tell all customers not to leave their home or go to bed with any washer, dryer, or range working, turned on.

After you have repaired several appliances you will have your own system to follow, but remember it can and sometimes will be something as simple as a bad wall outlet, open fuses, a control set wrong.  Always make sure the customer has not adjusted settings on the oven clock wrong.  For example the stop and start time on the oven clock.  (This happens) If the clock control times are set wrong the oven will not start because they have set the control for the oven to start at a predetermined time and may not even be aware of this! Observing and listening to what the customer is saying will be of great help always.

Regardless of what kind/type of appliance you are repairing take a few extra minutes to evaluate the problem, what repairs are required and to what degree of difficulty (time and liability) the repair will entail.  For example: are the screws rusted, access to the part that needs to be changed, what is the general condition of the appliance and age.  By doing this you will be able to give the customer an accurate price of repair before doing the repair.  This is something I always do and I never have any problems with the customer by letting them know what the cost will be first!