1) You always focus a certain minion, ignore others. (Don't lose lane awareness either or u will get pwned).
2) There is always a pattern to the minion being hit.
Minion is : _______
3 of ur minions are attacking him at the same time.... so probably it will be :
U attack him just after ur minions are done attacking it on the 3rd time.
Sometimes ur lane opponent pushes the wave in a way that enemy minions die quick by your minions like a damage over time, that's just hard and it requires more practice, like also last hitting under tower.
3) You can also hit minions with full health which are non focused, but u may end up losing a percentage of the whole minion wave. this is the "something is something" farm mode, it's not the best of course, but u can do it while u are still learning how to last hit properly most of the time
4) Early Game ADC: for fed (lets say u just bought a BF SWORD) u generally take a minion full bar in two hits; for non fed adc (lets say berserkers greaves and a doran at most) it takes three hits; if ur behind it's most likely u are gonna take four hits or more on a full minion
5)Later in the game u get crits, or more damage and atkspeed and can take minions easier.
6) Mages farm with spells, it's a tough decision here most times for a resource champion, either you farm with your aoe capabilities or go in for the kill. Early game as a mage focus on last hitting carefully with ur little damage until u have enough ap to take waves with ur burst.
7) Try to always get the big minion, most times it's the same difficulty to get as a little one and it pays double. so if u see ur minions attacking it, u stop attacking and prepare for the last hit.
Im not exactly a pro at last hitting but these are some techniques i've learned. Some pros end up farming like crazy because they basically last hit everything, if you master this, it while dramatically increase your chances to:
* Be more useful than the enemy if you put that gold in a good use, like rushing core items faster and dominate the game.
* Stay up to the opponent in gold.
* Help you to make a comeback after being destroyed in the early game.
Good luck and i hope u continue to improve with these tips or others, and practice.