Tracing TVI to its' "entry point" can, sometimes, take you round in circles because of a forgotten step. This step-by step process guides you through that maze and leads to the answer or solution in a logical manner.
You will be asked you to do a specific action then questioned on whether the TVI has cleared or is still present (or some other question).
On answering the next logical step is given, the fault diagnosed or a solution given.
Some operations should only be carried out by people who know what they are doing and have the relevant schematic to hand, there are voltages/currents inside televisions that can kill.
Transmitter tests can be carried out by any licenced Radio Amateur.
External television tests can be carried out by anybody.
Internal television tests should only be carried out by qualified/competent persons.