Here are a few suggestions. First, your manuscript should look typed, not typeset. This is particularly important if you are composing your manuscript on a computer, where the temptation to use fancy fonts can be great. Use a 12-point Courier, which prints out at a pitch of ten characters per inch. This is too small.
Courier is a monospaced font, which means that every character is exactly as wide as every other.
It is far easier for an editor to detect spelling errors in a monospaced font than in a proportional font, and your primary goal should be to make things as easy for the editor as possible. With a monospaced font, there will also be fewer characters on each line, which makes your lines easier for the editor to scan. Next, you should leave nice wide margins all around the page. Under no circumstances should there be any printing closer to any edge of the paper than one inch. Always double-space between lines.