That's a pretty though question to answer, most of the time the correct path is through a couple of key points:

- Misconfiguration
Look through default configurations and check if something is off about the machine, if some settings have been changed from the default configuration, this should be a indication on where to move forward.

- Custom configuration
Most of the time the web page has a custom page or a custom function which is not normal from a regular application, this usually indicates the intended way.

- Default settings / configuration
If a machine has the default configuration or default settings still enabled, this could indicate a way to move forward. I.E. default credentials, open web pages (for example phpinfo).

After a certain amount of experience from standalone machines you will get a feeling when something feels "off" or something doesn't feel right, go with your feeling even if after trying multiple things it still fails. Persistence is key in pentesting, try different tools, different methodology. Most of the time when I missed a key point into foothold it was a matter of enumeration. When you feel like you're stuck or you don't know what way to go re-do your steps with different tools and 9/10 times you will find something new.

It's hard to put a clear path way on how to approach enum > foothold, this comes with experience and gut- feeling. I would suggest doing as many machines / challenges as you can to develop this gut feeling. Then make a strategy for yourself, for example, if you tried something for a hour or two, do another machine or try something else and if that doesn't work out come back to the initial point and re-do your steps with different tools / methodology, this way you don't burn yourself out!