Proper Form Is Priority

Lately I've been finding myself more and more aware of whether or not my form is lacking during my workouts. I have no problem pushing myself through brutal cardio and HIIT routines but when it comes to lifting weights I feel relatively inexperienced. I find I keep having to watch myself in the mirror to see if my form looks the same as in the Fitness Blender videos, and even with the help of the small mirror I have, I still find myself questioning whether I really am working my muscle groups in the proper method.  I'm slowly learning that proper form is far more important that lifting heavier so I'm really going to try hard to best educate myself over the next little while. 

I decided to start with something simple, ie only body weight involved. Here is my plank:

I give myself a 3/5 for this one based on the following points: 
-straight line from head to toe -almost (butt needs to be a touch lower) 
-elbows under shoulders - check! 
-no weight in forearms - almost (I was getting tired so a bit shaky) 
-feet pushed into ground/quads flexed - check! 
-shoulders press up -almost (again very shaky) 

What do you guys think? How do you rate my plank form? I'd also love to hear your tips for performing proper form for any workout move! 
-Linds