You can search with following keywords in popular Search Engines like Google and Bing.

Your Keyword “guest post”
Your Keyword “write for us”
Your Keyword “guest article”
Your Keyword “guest post opportunities”
Your Keyword “this is a guest post by”

Find relevant blog in your niche and Contact webmaster with "Nice Customized Pitch" and your blog idea.

To post only on real sites that do not run too much guest posts, and that are in a suitable niche. Preferably you should also know the site owners, or have some sort of connection to them.

Avoid the IM variant like the plague.

i preferred PBN on guest posting. make your own pbn and ranks KW

The 'best' way to approach the guest posting strategy is to build a relationship with the site owner / content manager first.

Then, you agree on a content that is beneficial for both of your goals.

First you need to search site according to your niche with different query string .Send them request if they agree send content to webmasters.they will read content and approved . If you did not find site there are many instant guest posting sites.

Create quality content and include 3 to 4 keywords in your blog. Submit on high pr guest posting sites.

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Contact the editor
Wait for approval
Submit your guest post
Publish your guest post