The place to use the first instance of your keyword phrase on your page is where the search engines read first.
Most code starts at the top of the page in the header then progresses to the left column, the main part of the page or "body", then to the right column and then the end of the page. (The footer).

This is the way a search engine "bot" reads the page.

As SEO is the process of writing for people, start your page content with a question that is highly relevant to your primary keyword phrase.
After opening with the question, present information on the solution.

If possible, keep you page to one set of keyword phrases.
Use your keyword phrase ONLY WHEN NECESSARY.
I cannot stress this strongly enough.
As and example, if you have a series of articles relating to your keyword phrase then place them under a heading that incorporates that phrase. Do not list each article with the keyword phrase in the title.

I have seen owners of sites with a main page content of about 500 words using their keyword phrase 60 times, and then wondering why they are no longer showing well in Google. (Google tightened its keyword "stuffing" algorithm earlier this year.)
You can use it several times using a modifier if the modifier is relevant.
IE: SEO being the primary keyword can be modified by using marketing, work, explanation (of), etc.