After some research, I like a garden and I want to book it!

I log in or sign in if it's the first time.

I click on the garden I like:

On the right side of the page that will appear, there will be a booking section, I fill it out (I don't forget to tick breakfast if it is available and if I wish to add it) then I click on "Request to book":

If I can't select certain dates (which are grayed out), it means that the host availability dates don't match with when I would like to come. 

A window will pop up with a summary of my request, a space for payment and a tab to start talking with the host. From there I can introduce myself to the host with a short, friendly message to present my request. Then I click on "Request to book".

It is done, I just have to wait for the host to answer, he has 5 days for that and I'm notified by e-mail when he does :-)