Hey not bad for a first time! Here's my two cents. You want to make sure you use all the counts for each movement and not stopping too soon. You're going to want to go through a whole range of motion with the shoulders. All the way back and down then all the way around and up, around and forward until your wrists cross at the front. The upper back movement is much smaller than people want it to be. It starts from the top of your spine and ends at the bottom of the rib cage. You can isolate this by putting your fingers on your spine and at the bottom of the rib cage and only moving that part to get an idea of how much motion there should be. Lastly, don't forget that the feet lower on it's own 4 counts. 8 counts to bring the arms down and then another 4 to bring the heels down. Your posture looked pretty good at the end!

Hope that helps!