Before beginning, generate the code required to implement your button: see here for info (part 1).
Login to your Jimdo account
Open your site menu, then click "Settings".
Select "Edit Head"
Copy the head tag script and paste it in the appropriate area. It should look like this:
Save your changes.
Then, go to the page you want the button to appear on.
Click "Add element" in a section where you want the button to appear.
In the "More elements and Add-ons" section, select "Widget / HTML"
Copy the body tag script and the button code and paste them together here. It should look like this (booking widget v2):
Save your changes, then test your button.