Guest post backlinks are great to increase the ranking but remember do not do a 'guest post' on spammy websites. some of the ways to find 'Guest Post' sites in Google to increase your website ranking.
Here are some easy ways to find good relative websites on Google for a guest post. For example, If you want to rank your website on 'Blogging'
Blogging + “Guest Post”
Blogging + “Submit Guest Post”
Blogging + “Write for us”
Blogging + “Submit an article”
Blogging + “Contribute to our site”
Blogging + “This post was written by”
Blogging + “Guest Author”
Blogging + “Become An Author”
Blogging + “Suggest a Post”
Note: You need to write according to 'search intent' on those Guest Posts, or else all the hard work will result in nothing or very less benefit. Content Search Intent is the key in 2020.