Internal linking is the part of search work you control completely, and Terje Sannarnes gives it a whole post rather than a footnote. How your pages point at each other decides what a search engine believes the site is about, and it costs nothing except attention.
The link building run continues from there through earning links elsewhere, through blog commenting treated as still worth doing rather than dismissed out of hand, and through the plainest warning of the set: stop overusing your keywords, because the practice that once worked now marks a page as thin.
There is a genuinely useful post on choosing a company to do this work for you, written for somebody not yet able to judge whether they are being sold something.
He writes about mental and physical health for people running their own businesses, and about reselling hosting as a line of work.
Read the internal linking post and the keyword overuse post together. Those are the two changes you can make this week without anybody's permission.