Note: You might get a "Customized content!" notification to remind if there is customized content in your post. This is just to remind you that you don't accidentally overwrite your account-specific content of the posts.
To customize: Open Hookle and click "+" in the bottom navigation bar to create a new post or open existing post to edit. Now you can type text and add images. This content is copied automatically to all social accounts you've selected to your post. Start customizing by selecting the customize at the bottom right corner. Select any account for customization by swiping right or left. You can modify, add, or remove text and change pictures per social account as you wish.
You can also select easily accounts you want to post your message to by clicking the round blue "add account" symbol in the top left. Note that, you can currently select only one Twitter/Instagram account at a time. Twitter won't let third-party apps to post to multiple Twitter accounts simultaneously. Customization is available when at least two accounts are selected.