Once you’ve selected the symbol, look in the Properties tab; you should see a dropdown menu with the label “Color” that says, by default, “None”. This lets you set the brightness of your selected shape (Brightness), adjust the color (Tint), adjust the opacity (Alpha), or adjust all three at once using the Advanced settings options.
Select “Alpha” (make sure that you’re in frame 1 when you do this). When you do, a new dropdown will appear that opens into a percent slider. This works exactly like the Alpha settings on the color mixer. Move the slider to 0%; then go to frame 24, and repeat to set the opacity for that frame to 0% as well. Then on frames 12 and 13, rinse and repeat—only now you’ll have to set the sliders back from 0 to 100%.
