The second way of improving your Domain Authority is backlinking to other websites that have a high Domain Authority. If a website that has a high DA score has you as a reference or source of information on your website, this indicates you are a trustworthy website.
When you are starting off, backlinks can be very difficult to get. One way that may work is writing content for other sites and in exchange asking if they would include a link in the articles.
Some of the best websites to get backlinks from are .govs, .edu and wikipedia. Government websites, educational institutions and wikipedia are all websites that Google deems to be a credible source of information.