Rather than play with fonts, etc., for the headings, while you type up a document, simply mark-up a heading, as being a “heading,” or “sub-heading.” Likewise, mark-up the body text as being “body text,” important items as “important,” comments as “comments,” etc. Then use the style editor to control how all headings are displayed (what fonts they use, whether they’re bold, underlined, etc.), and the same for anything else that’s been “marked-up.”
(“Marking-up” means to give special meaning to some content. Generally, you’ll do this by highlighting something that’s different from the rest of the page, e.g. selecting a paragraph with the mouse, then pick a special style to apply to it from a menu.)
Now, you can modify those particular things from a central place. You can re-style the entire document, simply, on a whim (just to see how it’d look, with a different style), or to resolve some problem (such as headings not looking significantly different from the rest of the text), and not have to retype anything in the document. Some editors will even allow you to save the styles separately, so that you can use them with other documents, making it very easy to have a series of documents using a consistent style.
Marking-up the content in a semantic manner, also means that the editor can easily process the document to produce indexes, and tables of contents; as you’ve indicated what the headings are, and other points of interest.
Although this method may seem complex, and time-consuming, it’s really not. While authoring a document, you can concentrate on getting the content enterred (doing little more than just typing the words), then fancy it up, later on; experimenting with the “style,” seeing how the document looks, as a whole, presented in different ways.
Often, the only way to style a document in a good manner, is to play with its style when you have a large portion to look at. This is easy to do when the page styling is controlled from a master place (i.e. the style editor), but fiddly when you have to re-edit each special part on individual pages.
Incidentally, this technique is how this page is authored, along with the rest of the pages on this website (headings are marked as being “headings,” likewise for examples, explanations, comments, emphasised words, etc., and styles are applied to the headings, etc.).
Get a ruler out, and work out whereabouts you want something printed on the paper, and set margins into the word processor using real measurements (do this for objects on the page, and the overall page margins, etc.). If you want something 3 cm down and 4 cm across, then put that information into the word processor, and let it do it for you (it’ll probably get it right, first time, if your software is set up correctly). This is much easier than test printing something, moving it a bit, doing another test print, then doing all of that again when editing something else in the document disturbs its placement. At the very least, you’ll go through less trial and error cycles.
Likewise when you want to duplicate dimensions between different parts of a document. Look at the properties for the one you want to copy (e.g. “2 cm” margins), then put the same information into the properties of the thing you want to match (directly type the measurements into the styling controls, rather than try dragging margin pointers to the same place—you'll probably only get somewhat close to the right position that way). Unless you have a formatting cut and paste feature to do this for you, or apply the same style to both objects, this is the only way to get two things the same as each other.
Trial and error might get close, but is often annoyingly differently when it comes to printing, and very time consuming. Especially when something undoes all your efforts and you have to fiddle with things all over again to get back to what you had before.
If you were to write a document, periodically making some words bigger, smaller, or in a different font, then changed your mind about it, later on (perhaps, because you find out that what you did didn’t work), you’d have to re-edit the entire document, to modify each section, hoping that you don’t miss something.
It’s also difficult to be consistent if you do something a bit elaborate (like making all headings a different size, bold, and underlined), if you forget some aspect of that style (such as which size font you used). As well as time consuming, as you sit there trying to decide how to style some aspect of the document, instead of writing the thing.
Being consistent is important for making a document coherent to read, as well as making it look good. Things look better if all the pages are styled in the same manner, and it’s far easier to read. Especially if you’re creating a document with intermingled comments and demonstrations (styling them to look different than the main body text makes them easier to locate, or easier to skip past, to suit the reader's needs).
The size of blank spaces depends on the font being used, and the characters and words around them, as the computer will adjust the sizing of the spaces, to fit text across the page. Also, if you change fonts between lines, you may change the size of the spacing, in places that you don’t expect (and can’t immediately see).
This means that when trying to indent lines using blank spaces, the lines may not indent the same distance, even if they look similar on the screen (remember, the screen is just showing an “approximation” of the page), and you may not be able to “precisely” position things where you want, either (using blank spaces), as you cannot make tiny adjustments. Likewise, for playing with the positioning of things within a line, using blank spaces. Even worse, if you edit a line, everything else changes around the edit (you aren’t typing on a spot, you’re typing between other things; your typing will modify whatever’s around it, particularly what’s after it).
Likewise, the spacing between lines is affected by various things (the fonts, etc.), in ways which mightn’t be clearly obvious on the screen; and places where page breaks occurs can be hard to predict. One edit near the start of a document could re-paginate an entire document.
Do not spend two hours customising a document that should only have taken fifteen minutes to type. Type it in, then fiddle with the overall styling of the finished document with the style editor, else you'll be wasting a lot of your time in a multitude of ways:
Most obviously, the amount of your time you spend working on it (typing, retyping, adjusting, retyping, etc.).
Nobody else will be able to tell that you've spent all that time, and it's almost certain that they won't be able to see any worthwhile difference between a document you've spent far too much time on, and one that only had as much time spent on it as was needed, even if you point it out to them.
Nor will other people really care that you slaved over it for so long, if anything they'll think you're a twit for trying to ingratiate yourself by unneccessarily doing extra work. Nor should they have to care—if you want to waste time, that's your affair. Going fishing for praise, and trying to make a martyr out of yourself, over what's quite an ordinary task (typing something) is a very irritating habit.
On the other hand, if you type it all in then fiddle with it, you've got the whole document finished so you can use it if you need it, even if it looks a bit plainer than you'd like. And you can use your spare time to make it look better.