What is the best possible way to increase overall authority of a site?

[danni]

You can use Quora as part of a quality backlink strategy.

The idea is very simple you just need to have content that answers questions in your niche. Then, when you see that question you can answer it and link it to your content.

Another best way I found for myself is using HARO, It is a backlink outreach service that connects journalists to sources and provides you with the rare opportunity to get high-quality links.

If you remain consistent in your efforts then gradually you will be able to increase your overall DA.

Read before your post. OP didn't even asked about DA.