When writing a reference for someone on YJ, it's important to note that it'll be a big part of their profile. The reference you provide can be shown on their public profile, and can be seen by employers to which they are applying.
A great reference gives insight into the work that you have witnessed the person you are providing the reference for do, as well as the quality of the work.
I had the pleasure of working with Sam at YunoJuno, where he worked on the design of the product. He has a strong understanding of visual design, and was able to work well with the rest of the development team to improve the look and feel of our outputs. I would be more than happy to recommend Sam to anyone looking to hire him for their next project!