Step 1: Navigate to your Deck and then select the "Tools" dropdown menu from the top. From there, select "Preselects" which will bring you to a new menu.
Step 2: Create a Preselect by giving it a title and then create and edit the slides.
Step 3: Click "Add Slide" and navigate through your Master Deck to select the slides you wish to include in this.
Once that is complete, you can save and create that preselect.
Adding Your Preselect as an Option when Creating a Presentation
Step 1: Navigate to the top bar again and click tools, and then the presentation flows.
Step 2: Create a flow and name that is similar to the Preselect that you created.
Step 3: Select "Preselect" from the Presentation Flow. You will then be able to select the Preselect that you just created in the previous steps.
Step 4: Create a new presentation and then within the Presentation Flow option, you can now select the flow that you just created.