With one finished outfit and your shoes on, stand in the mirror and take in the entire length of the outfit from the neck to the hem without changing anything yet. The proportions are easier to assess when all items are included, because a piece which looks fine on its own is often improved or ruined by other pieces, such as trousers, a skirt, a jacket, or shoes, which might have changed that proportion entirely.
Rather than deciding whether the outfit is good or not good, instead notice what the eye notices first. The eye may be drawn to an extra long top, to a weighty shoe, a jacket finishing in a poor location, or a belt which divides the outfit too severely. The first impression shows where a balance is best, or most awkward. You do not want the perfect shape here, you want to understand how every single element is playing off the others.
To start, notice three elements from a first mirror view. Notice where the shoulder line is, to see whether the top end of the outfit is structured, relaxed, or too wide. Notice the location of the waist, whether it defines a center point in the outfit or if everything blends together. Notice the length of the hem, whether of a trouser, skirt, dress, or over layer, and consider how it affects the lower half of the outfit. Notice how a small adjustment in any one of these points affects the entire outfit.
Now, make a single change, then look again. Tuck the front of a shirt to show more of the waist, or unbutton a cardigan, change a belt to a different width, roll a sleeve, swap a flat shoe for a slightly heavier one, or take off a long layer. Leave all the other items unchanged so you can tell what the change actually did. If you make five changes at once, it will be very difficult to know which change improved the overall balance.
When you begin, it may feel easier to focus your eyes only on the item that feels least comfortable in the outfit. You may check whether a particular neckline is working for you, only the neckline of a top, or you may decide to check only the trousers because of the waist fit. However, proportions always relate to how the items fit together. A wide trouser leg can look very beautiful paired with a shorter top but look lost on a longer, looser sweatshirt. A dainty flat shoe can balance a narrow hem well, but appear too tiny beneath a heavier shape.
Looking at your outfit on your body in photographs can also be useful in place of using the mirror. Take a front photo and a side photo while standing in a natural posture, step away, then take a second set of photos after the one change. Notice what the eyes notice, whether there is clearly defined top, middle, and bottom, whether one of the pieces feels much heavier than the rest, whether the shoes seem like they would support the length of the hem of a trouser, skirt, or dress. Consider whether an over layer finishes in a place that works well with the outfit overall, or whether it cuts the shape too sharply.
Make your last mirror note one clear observation, rather than trying to make a general conclusion about your outfit. For example, write something like: “shorter top works better with these trousers,” “heavier shoes balance out this wide hem,” or “open over layer gives the waist more shape.” With this practice, the proportions of your outfits soon stop feeling so mysterious, and you will begin to recognize how well the outfit works before feeling the need to start over.