There are a couple of different ways to work out the word-count on a piece of work: the easy way and the hard way.
The hard way, which is also the more accurate way, is to set up your word-processor so that each page carries an average of 250 words. Most of the online instructions are seen for this don’t take into account the differences in standard paper sizes: in the US, the standard paper, letter size, measures 8.5 inches wide by 11 inches long. In the UK A4 is standard, which measures 8.3 inches by 11.7 inches long. What we UK writers lose in width, we make up for in length (and no, I’m not going to make the joke).
This slight difference in size makes it difficult to give precise instructions for the 250 method: the gist is, though, that by setting up your standard page template to use margins of roughly one inch all round, each line should contain an average of ten words. This won’t work properly unless you’re using a non-proportional font like Courier New, where each character takes up the same space on a line, rather than a proportionately-spaced font like Times new Roman, where characters like “i” and “l” take up a lot less space than characters like “m” and “w”.
Once you’ve worked out the ten-words-per-line part, you then need to set up your page to contain precisely 25 lines of text. As soon as that’s done, each page is going to average 250 words.
It’s easy enough to work out that once that’s set up, you have roughly 1,000 words for every four pages of text; and that if you’re aiming for 80,000 words—about the minimum length you’re going to need for a novel—you’re going to need to produce around 320 pages. You will have to make allowances for half-filled pages, but you're pretty much there.
The easy way is to use your word processor’s built-in word-count facility. Two clicks and you’ve got it. This is how I work, and it has served me pretty well. It might not be quite as accurate as the 250 method, but for me it comes close enough.
I’ve been told that editors prefer writers to use the more accurate 250 method, and I’ve seen all sorts of anxieties arise as a result. There's no need to worry. When I was an editor I relied on Word’s word-count and never had a problem with it. More important, in all these years I’ve never heard of a book being rejected because the wrong sort of word-count was used. Just get it as accurate as you can, and make sure that every single word counts.