You should be logging user’s actions throughout your program into some location. For me I was able to just keep a list of strings, but depending on your application you may need to store it locally on some kind of text file. Logs should also be prefaced with a time stamp and a tag that marks what file the log came from to make it easier to look up when looking through them. 

If your application calls for it, screenshots/videos are also good ways of keeping track of things. Taking a quick screenshot and saving that when you encounter something important isn’t unreasonable, or a quick video of a process works well.

Depending on your needs as well it can be important to log information about the end user’s  device as well, for example their IP/MAC address, GPU/CPU, RAM, current memory usage, screen , window dimensions, etc. Anything that can be related to your program.

Catching exceptions is also important. Whenever an error/exception is thrown your IDE will log this but how do you go about getting that exception? Simply surround your code with try { } then after add catch(Exception e) { } after the try expression, like below.

try
{
    StreamWriter sw = new StreamWriter("example.txt");
    StreamReader sr = new StreamReader("example.txt");
}
catch (FileNotFoundException e)
{
    Log("Could not find: example.txt, " + e.ToString());
}
catch (AccessViolationException e)
{
    Log("Access violation! " + e.ToString()); 
}
catch (Exception e)
{
    Log(e.ToString());
}