Here is how I do it. Albert Handell showed me and it takes a minute.
Extend the back/middle leg all the way and use it as a stand as you rest the front part of the box (the area between the two front legs) on your knee (I use my right knee).
Extend each of the other two legs, one first, all the way and let the easel stand on it (middle of the box still on your knee). The easel will now be using two legs and your knee. Finally release the third leg all the way. Done.
I found a video showing this way.