This ring holds colored gels by placing the gel inside the hinge and closing it. Two magnets will hold it closed.
The gel paper does not need to be trimmed, it fits inside the soft box with no issue with the overhanging material.
Inside the soft box, there is a separate interior diffuser held in place by four pieces of velcro in the corners, you can undo two and move it to the side to install the gel holder.
Pinch the two metal tabs on the gel holder and slot it into the metal ring inside the soft box. Release the tabs and the gel holder will sit firmly in place. It can still spin freely, as it is just held in with tension.
You can then reattach the interior diffusive material as well as the exterior diffusive material with the velcro.
Once all diffusion is put back in place, the soft box will now produce colored soft light.
Examples
By using the same color gels in multiple lights, you can achieve an even color cast
A common use of colored gels in studio lighting is to create a split toning effect, typically using a cool color contrasted with a warm color coming from opposite directions. Keep in mind that when using colored gels, you will notice a decrease in the intensity of the light's output correlated to how saturated of a color the gel is. The blue gel used here is a very deep blue, resulting in a lower brightness output, but very intense color.
Another example of a warm/cool split toning effect with a yellow gel instead of red
Using two cool tones or two warm tones tends to have an additive effect as opposed to the more sharply defined contrast of a warm and a cool tone gel
You can create dramatic effects by using the colored gels without diffusive material and adding in white light to illuminate the subject from the front. In this case, the red and blue lights are placed facing one another on either side of the subject and the non-diffused white light is positioned high and to the front of the subject off to the left side. Note that the lack of diffusion creates very crisply delineated zones where the color of one light is blocked by the subject's features or clothes.