First: why should you link audit competitors?
Second: who are your competitors and how to find them.
Third: doing the link audit.
Fourth: use the link audit to build links to your own site.

why should you link audit competitors
There are a few good reasons to do a link audit on your competitors site, the biggest reason is to build a database of the common links between all of your competitors sites that you don’t already have to your own site so that you can go and add new links to your own site :D
It will also teach you how to find good links and identify bad ones

Who are your competitors?
Your competitors are everyone who ranks above you in each result page you are in on each search engine.
You have a lot of competitors.

How to find your competitors:
This bit is easy enough, if time consuming: take every url in your site and every set of keywords that it ranks for and take a note of the url of every person who appears above you. In practice you might want to concentrate on your top ten or so performing pages and their top few keywords and audit these first.
You can find your top ten terms in google webmaster-tools.

Goto google webmastertools > search traffic > search queries

There will be a ‘download table’ button, click it :D

You can find your top performing urls in google analytics.

Goto google analytics > behaviour > site content > all pages

This report will show your top performing pages for the dates selected in the top right corner.

Once you have the list of terms from google webmaster-tools, you need to type each into google.
The sites / pages that appear are your competitors.
It would probably be best to copy these urls into a spreadsheet.

Doing the link audit:
Once you have your competitors you need to plug each of their deep-urls (and each of their home pages) into ahrefs (or majestic sep tools, or open-site-explorer) and then take a note of each of the back links that your competitor page has.
You will now have a massive list. This list is your ‘potential back-links’ list.

Use the link audit to build links to your own site:
What you need to then do with your potential back links list is to visit each page in it and see how your competitor got a link – some will be comments, some will be forums, some will be guest posts, etc.

Then its up to you to figure out how you get the same link..
eg, if the page is a blog page and your competitor has commented, then join the conversation…