How do you contact other bloggers to ask them for guest posts?

Most bloggers will have either their email ID or social media handle on their blogs. You can simply contact them through these channels.

But before you do that be well versed with the blog for which you are asking someone for guest blogging. Follow the blog genuinely for, say, few months to get the hang of the blog content.You can then pitch an idea (or even a blog post ready) on a topic which hasn't been explored or less discussed on the blog. Be genuine with your comments. People can smell fakers from a mile.

I'll you the contact details provided in the blogger, if I didn't find it; I will look for the blogger through google, I may find him on facebook or any other site.

One of the best ways to do it is to join a network of writers and bloggers and see what they have.  If you find some of them share your genre, you may invite them to guest post in your site.  You may even want to request for a recommendation from them.

Here is one sample of an invitation I received from another writer, and I quote a portion of his invitation to me:

"... Always looking for interesting and pertinent guest blogs/articles and happy to return favour personally or by introduction to a group member...."