Guest posting is the same as sales pitching. You need to do your research before you reach out to the guest and pitch the favour.
If you're immersed in the culture of your blog niche, it should be easy. You already know the important figures in the field and how the community interacts.
Those are not enough knowledge, though. You have to research as many topics as possible. You must have enough selection before choosing which to pitch to your target guest.
Of course, that also means you have to research more about the guest. Study their content and branding thoroughly. You can't miss out on little details. Those are the usual points of entry in these cases.
Do your research as if you only get one shot at this. That could be true depending on the standards of the guest. That reminds me, you should have information about their standards as well.
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I don't mean just an idea. See if you can speak with someone who has worked for them. That always helps. You can inquire if they can put in a word for you.
Researching and networking will be your initial step in this process. For the sales pitch, find the perfect line between professional and personal.
Your guests would be thrilled to do a favour for your blog if they sense you're fighting for the same cause or working on the same creative goal. They also need assurance that you could be as distinguished as they are.
Once you've sealed the deal, you can relax and let them do their magic as you assist them all the way, of course.