From the Pages screen (Admin Tools > CMS Configuration > Pages), click the page you want to edit from the tree navigation. From here, you can edit an existing block by clicking the button. You can also add a new block by selecting the zone where you want the block to go from the dropdown menu on the right, then clicking ti ti-circle-plus Add Block to Zone to display the Add Block window. Note here the option to add this block to the Page, Layout or Site. Just like adding a block to your external site, you need to decide if you want this block to only appear on the current page, on any page using this layout or across your entire internal site. The Layout and Site options are best used when adding a block you know for sure belongs on every page, such as a header or footer. Otherwise, it's best to use the Page option.