Before HOW? let's have a look in to WHY?
Anyone can think that our website is up and running well why do we need to do a site audit.
It's not that the website up and running won't have any issues. Doing a complete site audit will help you identify hidden issues on your website which you didn't see in the first place.
By performing a regular site audit you can make your website reach full potential.
Conducting a site audit will help you determine whether your website is fully optimized for search engine traffic, has any broken links or pages, is your website loads faster, is mobile friendly and has high-quality content.

There are a lot of site audit tools available on the internet;

Technical issues:-
To list a few of the technical issues that your site audit tool will show;
1) robots.txt
2) Website load time
3) Broken images
4) Website Https status
5) Broken links
6) Sitemap.xml
7) Checks for duplicate content

robots.txt File:-
robots.txt file informs search engine crawler which pages to crawl and omit. This is used to avoid overloading of unwanted requests.
robot.txt file is mainly used to manage crawler traffic on your website.

As mentioned by google if you are using a web hosting service like blogger or wix you may not need to edit your robots.txt file directly. Instead, your web hosting provider may provide you with some other options to inform google about which page to crawl and not to crawl.

Google bot and other respectable web crawlers will obey the instructions on robots.txt file but other crawlers may not obey it, therefore it is better to use other blocking methods also.

Website Load Time:-
Website load time is very important because no one likes to stay on a website that takes too long to load fully.
Google has a wonderful tool which will provide you insights on what should be done to increase your website speed.

Broken Images:-
The broken images will negatively affect the user experience. Search engines won't be able to index these images. These images could have been moved, replace or deleted. If you are wondering, how to fix this issue replace, remove or delete the image.

Website HTTPS status:-
One of the most important assessments in any SEO audit is determining what HTTP Status Codes exist on a website.
Types of status codes;
1) 1xx status codes are informational codes.
2) 2xx codes are success codes.
3) 3xx redirection codes are redirects.
4) 4xx are any codes that fail to load on the client-side.
5) 5xx are any codes that fail to load due to a server error.

Sitemap.xml:
A sitemap will help the search engine crawl all the pages on your website without omitting them.
A sitemap is a file where you provide information about the pages, videos, and other files on your site, and the relationships between them as mentioned by Google.
If your website is a small frame website with fewer pages and images you may not need a sitemap, but if your website has a large number of files and interlinking blog posts you need to submit your sitemap.xml to help Google crawl through your website.