You don't want to over do it.
Generally, as far as i know - This would be reasonable
Put it In the title.
Put it somewhere in your first paragraph.
Put it somewhere in your last paragraph/couple of senstences.
Put it in once or twice (maybe 2 or 3)in the middle paragraphs.
and in your tags !!
Don't go any more than that -
You can of course be repeating this process for more than 1 keyword.
Another good tip is to bold your keyword once and put in italics once also - helps the google bots and spiders etc know they're significant words
Should be easy enough since u won't be overdoing it -you need to re-word where you're entering your keywords so it reads well rather than looking like your stuffing keywords in for the sake of it.If this is proving difficult it may be easier to change your keywords slightly if you can't re-write the relevent sentences.