The biggest hindrance to forearm stand, to me, was that my shoulders were tight. So, it helped me to *open my shoulders with eagle arms first and foremost.*
Then, if the shoulders are still tight in a forearm stand, the hands will come together and the elbows will flare out, which isn't good alignment, so I put a yoga block (or box in this case) between my hands and straps around the elbows to prevent them from flaring out. (The straps shouldn't make your elbows narrower than shoulder width though! No reason to make it harder than it already is!)
With the elbows strapped and the hands set around the block, go into dolphin downward dog (downward dog on your forearms), raise one leg up into 3-legged dolphin, then kick up to forearm stand against a wall and hold for time.
This will help open the shoulders and help you understand how to press your forearms/shoulders into the ground to find your balance. It's really weird at first finding that engagement but then it becomes nice.