Some people are fortunate in being able easily to make graceful letters, to space their words evenly, and to put them on a page so that the picture is pleasing; others are discouraged at the outset because their fingers are clumsy, and their efforts crude; but no matter how badly formed each individual letter may be, if the writing is consistent throughout, the page as a whole looks fairly well.
You can make yourself write neatly and legibly. You can (with the help of a dictionary if necessary) spell correctly; you can be sure that you understand the meaning of every word you use. If it is hard for you to write in a straight line, use the lined guide that comes with nearly all stationery; if impossible to keep an even margin, draw a perpendicular line at the left of the guide so that you can start each new line of writing on it.
You can also make a guide to slip under the envelope. Far better to use a guide than to send envelopes and pages of writing that slide up hill and down, in uncontrolled disorder.