SEO/organic marketing is all about content. Link building involves getting links (Called backlinks) from other sites, and authority sites will only give you a backlink if you offer them good content in return. Guest posting is about writing relevant contents for third-party sites for link building.
If you are not sure of how to proceed with that, you can hire paid guest blogging services, which will take care of the basics. From finding topics for your niche to selecting authority sites and creating engaging content, they know how to entice the audience and take your brand to the next level.