You can build high page authority to a website by following White Hat & Grey Hat SEO techniques.

Make sure your website doesn't contain more than 3 layer linking system, as too many internal links might not help the website to rank better in Google. You can use any Ahrefs.com, open site explorer or majestic seo to find the top authority pages within your website.

Can someone pls explain step by step?

To build more back links and content should be unique to increase the page authority...

You can improve your page authority by giving the following features in your site

High-Quality Content
Reliable, Functional Backlinks
A Robust Web of Internal Backlinks
Regular Social Media Interaction
Fresh, New Content All the Time.

The best way to influence your domain and page authority is to focus on your overall SEO. This included every aspect, from the content of your site, on-page SEO, how often it gets updated, the pagerank, traffic and your link profile. There isn't really a 'quick' way to change it but you should focus on getting quality content rather than quantity. Articles should be over 500 words and relevant to your niche. What's most important though is your link profile, so if you're going to focus on something then sort that out and try to get as many relevant backlinks as you can get.

Page authority is the measure of trust a webpage has in search engines. It can be improved by good content update and also by building backlinks from other authority webpages.