The OP asked for advice when starting to use manual mode, so that means advice for a new or beginning user of manual mode. I'll add another comment to this thread based on that assumption, that of a new user.
First, stay away from extremes on the lens aperture and shutter speed. Speeds of 1/8000 and 1/4000 second are seldom needed, and they force you to use very wide apertures like f/1.4 or f/1.8 or f/2 quite frequently. Similarly, using f/stops like f/1.4 and f/1.8, and even f/2.8 on an f/2.8 zoom--these extreme aperture settings bring with them a number of 'issues', like softness, lots of vignetting, shallow depth of field, and a need for exceptionally precise and exceptionally accurate focusing.
In either AUTO ISO mode, or in manual ISO selection mode with Manual exposure, one is basically setting either an f/stop, or a shutter speed, and then moving the opposite control until the meter is zero'd, so, don't start out by setting the lens wide-open, or the shutter at its fastest, or slowest speed. Similarly, in most cases, stay away from the smallest lens apertures like f/16. Instead of the extremes, look at ways to get moderate aperture values and moderately fast shutter speeds, like f/stops of f/4.5 or f/4.8 or f/5.6, and shutter speeds of 1/640 to 1/320. In these moderate ranges, the lens will be very sharp, focus will not be ultra-critical, and shutter speeds of 1/640 to 1/320 second will stop the majority of hand movement, camera shake, and normal human activity, and you'll get good images most of the time.
The above is advice for outdoor conditions in decent light levels. When the light levels are very low, hand-held photos call for wide f/stops like f/2, and slow speeds, like 1/30 to 1/60 second, OR they call for timed exposures made while shooting off of a tripod, with exposures ranging from 1/8 to 30 seconds.