First, you may want to check the docs of the original Hatch theme. The current document only covers the differences and extras over the original Hatch theme.
To create a short resume text to the left of the header image, navigate to Appearance → Theme Settings and add your text to the metabox “Author Info” (upper right corner of the screen). You can use almost any HTML elements and/or shortcodes – headings, links, paragraphs, … Whatever you want. Then click “Update Settings” – the blue button at the bottom of the screen, and you are done.
To fill the “callout” section (below the header image on the home page) with your own content, go to Appearance → Theme Settings and add any type of text, markup, or shortcodes, to the box “Callout” (bottom right).
Two switch between 3- and 4-column layout for the main content, go to Appearance → Theme Settings, and select the desired option in “Columns”.
Hatch Pro supports two post formats – ‘standard’ and ‘gallery’. You can set the format for each post from the post editor screen. The ‘gallery’ format provides a full-width layout, suitable for image galleries and large photos. It’s not that good for text, as the lines would be too long for pleasant reading. If you want to create a blog post with predominantly text content, you should pick the ‘standard’ format.
How to Customize
The official way to customize WordPress themes in general is through a child theme. Learn more about child themes here. Now that you know how to customize/modify correctly, get a head start by downloading a Hatch Pro Child theme from the Hatch Pro page.