Most SEO experts will say that Guest Posting or Guest Blogging is about FIVE TIMES better than creating the content on your own website. (This is of course, subsequent to building out your website amply - for SEO backinks it just plain works!)
Again, relationship building, branding yourself, branding your new company, valuable targeted SEO backlinks and long-term targeted website traffic are just some of the major benefits.
TIPS to be a HUGE Success in the Guest Posting Arena
Schedule some TIME exclusively for this type of website promotion.
Yes, it's marketing in a way. Yes, it's Off-Page SEO in a huge way. But, it's going to help develop your research skills, discover how your competitors build websites and further your all-around knowledge about search engines and networking.
Trending Topics
Discover ways to find out what the trending topics for your chosen niche are. You're likely doing this already to help create your own content for your own website. But, this is also valid for guest posting. Other websites that you're interested in posting on, just like you, are likely aware of the current trending topics.
Google Trends can help:
https://www.google.com/trends/
Also, use your social media platforms for current topics and issues to guest post on.
Submit to ALL Sizes of Blogs
Obviously, you'll want to begin on smaller blogs to gain to "street creds" and to use a references. The first one will be the most difficult. But, learn to walk before you run. You're gut instinct may be to approach the big boys out of the gate. I urge you to avoid this and start small. Like anything else, practice and hone your skills. There is an art to approaching a webmaster to built a rapport and exchange posts.