a) Click on Options on the left side of the main menu.
b) Then click on Options again.
c) You are automatically in the SEO section.
d) The subitem "Title/Description" is already selected.
e) On the right of the menu you can make your entries as follows.
1) Here you enter the title
2) Here you determine whether the general title should be displayed if the page has its own title and if so at which position.
3) The field for the general description of your website.
4) Here you can upload an image as a teaser. It will be shown preferentially when you link your website on other sites like Facebook or Twitter.
5) The preview shows you how Google will display your site in the search results. If your description is too long, the text will be cut off and several dots .... will be displayed instead.
Note: Choose the description very carefully. You have just 160 characters to convince people who found your site on Google that they are right for you. So don't waste the space with meaningless phrases but get to the point.