And a few more tips for practicing:
#1 Use a few yoga blocks to practice! Just place them directly under your shoulders (on the 'higher level') so you can better estimate the correct distance from your hands to your shoulders. Alternatively, you can place one block under your thighs and the other under your chest.
#2 The cobra asana is the perfect preparation for the chaturanga - it is often a little easier and strengthens the muscles for the chaturanga, so that at some point it almost works by itself.