First, in lane:
Pay attention to every minion, constantly check their remaining health and how many of your minions are attacking them. Watch out for harass and also check your minion's health for when to harass your opponent, but that comes secondary to securing your own CS. Missing 2 per wave is only 4/6 though, which would mean in 17 waves (10min) you'd have around 68 (around what you claim to have). Also realize that if you leave lane without pushing it hard and fast first, you'll miss some CS before you get back.
When roaming:
Take whatever CS you see if you're a carry. The ADC gets all of the cs generally (a mid laner won't just give you cs but no mid laner is going to whine that you're stealing cs as an adc). When a large collection of minions gathers somewhere, call that you'll take it and go farm it. Be sure to have vision of the enemy team if you're doing this though, nothing is worse than the enemy assassin waiting there in a bush for you and then killing you (not only do you feed a kill but you also then miss all of that CS).
Otherwise, just practice. Last hitting takes practice. A lot. You'll get better over time.