Their is a few things you should know, while you do a good amount of damage with this build the goal isn't to kill the enemies, its to leave them on a tiny amount of health so the adc gets an easy kill. By Using this build you do enough damage to do massive amount of damage but not kill the enemy, this is great for supporting by building AP your heal and shield from Q and E are alot stronger then you might think, I managed to heal half a health bar of most of my adcs while in laning phase.

You might wanna think of rakan like a boomarange your adc is throwing, you go in and the adc is your safe zone so keep them alive that is the key for this build you will not do enough damage to kill people your level, however you will hurt.

The combos are very simple for rakan however they change depending on the situation.

The engage is a simple combo of.
W AA Q
W in to the enemy if the knock up hits auto attack and follow up with a Q this can be done very fast. You will knock up the enemy activate your electrocute and deal a large chunk of exp to the enemy.

this is where the fight can change,
if the enemy team goes in you can exhaust the adc to minigate the damage or you can use it if they try to get away to slow them down remember your not trying to get kills just serve your adc easy kills with this build

if you see your adc losing simple E to them after using E remmber you have another E so don't be afraid to give some cheeky AA while doing this, your shield doesn't stack sadly.

If you W into the enemy team what I like to do is ult if their is a large amount,that way I get the move speed buffs from my runes and items and its very easy to get away just make sure to touch the enemies with your ult so that they get charmed by your dashing good looks. another thing is after touching someone with ult dont be afraid to through in a Q if you Q someone your W hit and you ulted your Electrocute will go off giving some random surprise damage to some unlucky person.

Other then that their is not much else to combos in this,

What you should mainly worry about is early game damage dealers like Draven, People who can easily 1v1 your adc while you W in are hell to fight against with this early, you shouldn't try to fight them until you are certian your adc can kill them because you will not do enough damage easpecially early game.

The next thing to worry about would be hooks such as blitz, naut, thresh they can simply hook you when you W in or hook your adc when you W in.

The best adcs are ones with strong early game for this build, that is mainly because you need to make sure the adcs have enough damage early game to secure the kill when you go in. For example if you went in and had a twitch adc while they could get the kill you need them get it fast enough before the enemy can heal and use abilities, however if you go in and you have a draven who chunks 1 or 2 axes after you go in they get the kill extremly easily.