Convert unlinked mentions of your brands to backlinks, by sending the domain administrator an email and asking them to do so. You can use Ahrefs Content Explorer for this.
Look for pages that mention multiple competitors, but not you. You can use SEMRush's Link Building Tool for this.
Get listed on relevant " best x in y " lists. Reach out to them via email and ask to be listed.
The broken link method. Find broken links that your target domains are holding onto, create content and reach out to them to replace the broken link with your website.
Create an infographic and allow people share them on their website by linking back to your site.
Respond to questions on Quora. And link to your website on your responds.
Leave testimonials on other websites and get a backlink to yours when they've posted them.
Use Internal backlinks. One of the most effective and quickest ways to promote a new page is through internally linking to it on your existing, already ranked pages.
Post comments on relevant blogs and forums.
Get on Help a Reporter Out (HARO) and get high quality backlinks from news sources.
Create Award Badges for people to put on their website and link back to yours (if you have a relatively well-known & authoritative website/blog).
Get Interviewed by other Websites.
Create an Industry-Specific Business Directory. Done right, it's a great way to gain quality and relevant backlinks quickly.
Submit your site to niche specific directories.