Once you're happy with your design, simply exiting the workshop will save it and create the detail block to build with. To select it, open up your Block Select screen ('=') and select your new design to a hotbar slot. You can quickly find all detail block designs by filtering for 'detail block' in the filter list. You can now start a new design in the workshop by pressing 'Y', or by building a new workshop. You can have up to 16384 detail block designs in one world!