First you should identify blogs that you know accept posts from guest bloggers. If you don't see guest blog posts published on that site then the odds of them accepting your guest blog post request is very small. You might get some takers, but you will get rejected a lot.
If you identify a blog that accepts guest blog posts, then spend a month on that blog interacting with the people on there such as the blog owner and people who comment on that blog. Talk with them so that they meet you for the first time and get to know you.
Once you have been talking with them for a month or so, then ask them if they allow guest blog posting and what you would need to do in order to get a blog post published there.
The more that person likes you, the more likely he or she is to allow you to publish a guest blog post. Some sites are easier to guest post on than others. If the site is harder to get a guest blog post published on, then there is a good chance it is because that site is a higher quality site. So, it can be worth the extra effort you have to go through to get published there.
If you just go around shooting emails to strangers asking if you can publish your guest blog post on their site, most of the blog owners are just going to ignore you and not even bother responding. It is kind of like someone walking up to your home and knocking on your door and asking if you want to hand them a five dollar bill in exchange for absolutely nothing. Most people will slam the door in your face. And some of them will cuss at you before slamming the door.
You have to give before you get, unless you find a situation where people are trying to build an entire website from guest blog posts. And those places do exist.
