When you start designing letterforms without any typographical knowledge, the obvious to way to go about things is to align everything the same way, make everything "perfect", but then you end up with something that is not balanced.
The example below summarize this perfectly, every element that you introduces into your design is different, this new element will influence the overall design and will also change your perception on the existing elements.
The square and circle have different forms, the difference between them is that the circle leaves more white space thus it makes the circle looks smaller compared to the square if not adjusted optically
Mathematically adjusted shapes
Optically adjusted shapes
The space within letters should be equal to the space between the letters
Spacing is one of the most important things in typography, you can have the most beautiful letters in the world but if they are poorly spaced, your design will look very amateurish.
Spacing shares the same principle of optical balance, instead of making things right you need to make things look right. Spacing your type mathematically will give you poor results, instead space optically which means making the space within letters the same as the space between letters.
Tip: When you space letters, instead of thinking about the space between the letters, think about the volume between them and make it equal (imagine there is air or water between the letters).
Geomtric type is not geometric
While geometric typefaces are called geometric, if you dig a little deeper you will find that they are not perfectly geometric, and this is fine because if they were they would not look bad.
Below are two circles, a perfect one, and an adjusted one, the one on the left looks better because of optical adjustments altough it is not a perfect circle, this is due to an optical illusion.
When you have two bars, one horizontal and the other one vertical with the same weight, the horizontal one will look heavier compared to the vertical one.
Left: Adjusted circle. Right: Perfect circle.
When you have two bars, one horizontal and the other one vertical with the same weight, the horizontal one will look heavier compared to the vertical one. Think of people who are tall and fat, they are still fat but they look less fat than people who are short and fat.
There is another illusion, if two strokes intersect they will create a surplus of black, thus you have to make one stroke a bit thinner than the other to balance everything out.
Left: The strokes have the same thickness. Right: Adjusted horizontal stroke.
Anatomy of typography
Letters are just like humans, they have an anatomy, knowing the anatomy of typography will help you understand how each letter is built and how each part influence your design, you can then play with these small elements to create a whole letter.
More detailed article can be found here: The Anatomy of Typography
Italics are not perfectly slanted
Longer forms need to slant less than short forms in order to give the appearance of a consistent slant.
True italics give the appearance of having the same slant, this is what is not recommended to slant a regular font, the slant angle of letters will appear inconsistent, even though it is.
The letters 'f' and 'd' have less slant than the other short letterforms.
It's all black and white
In the end of the day, typography is all about black and white space, you have a white canvas and you paint with black.
You decide how much you add and how much substract, your negative space will influence your positive space and vice versa. It is that simple and this what will make or break your design.
Tip: Work in black and white first, it will help you criticize your work and will keep you focused on the form of your design.