I'd suggest learning to use the flash in eTTL mode with the camera in Manual.
Then your thought process goes something like this:
- Decide if you want the flash to be the main light or a fill-light.
- If it's as the main light, use the manual camera settings to set an exposure which works for the background
- Take a photo with the Flash Exposure Compensation (FEC) set to 0.
- See if it the subject is exposed correctly, and if not adjust the FEC
Alternatively:
- If you want to use the flash as a fill-light...
- Set your manual exposure so that the subject is exposed correctly
- Take a photo with the Flash Exposure Compensation (FEC) set to -2.
- If you're not happy with the amount of fill, adjust the FEC up or down accordingly
That's the basic process I work through when using the flash. With practice, you'll get better at judging what FEC you need in different situations (e.g. with a white subject +1 FEC is a better starting point.)
You'll also want to consider where the light is coming from. On-camera flash can look quite harsh as a main light, so consider bouncing it off walls/ceilings or getting the flash off-camera in some way.