1. Create the layer you want to make a clipping mask on.
2. Hover your mouse between the two layers (as shown in image above)
3. Whilst holding down the alt (or option) key, click.
4. Ta-dah clipping mask created.
Learned it from this website, and it took me a few attempts to actually figure out how to do it. When I finally got it, my initial reaction was: "Wah lau liddat in between the line click also can ah."
This will save me whole seconds whilst at work. Hahaha.
By the way if your alt/option key is spoilt, or you don't want to click between layers, you can always right-click and select add clipping mask (which is what I was always doing in the past, all my wasted seconds!). You can also just press Command-alt-G.
This should be a new section of posts called things I learnt at work today.