Well as everyone says build quality backlinks. build backlinks on high DA websites because their link juice is one of the factors to increase DA but only create do-follow links.

The best way to do an audit is through any SEO tools. because tools show all the error you need to fix like H1, Meta Description missing, or if there is any 404 page on your that need to be redirected. apart from that, you can take the help of an SEO agency or a freelancer to do a professional SEO audit.

My suggestion would be to do it by yourself if you have the access to paid tools if not then take a help of a third party

@zeitnot Well, guest posts are also paid and free too. Top sites required quality content only and on other hand paid links are where you insert a link in an existing article/blog. Many websites accept paid guest posts only