here i will give you tips for ypur start as a Rakan Support, on this chapter we will look for know how your champ works and how do basic combos:
for first we will look the basic combo, you can use your Battle Dance on ally near the enemy champion to have more odds of hit your Grand Entrance, later of hit your Grand Entrance you will use your Gleaming Quill, this for make more damage and heal your ally beacuse you will use the second charge of your Battle Dance (if you used it to move to your enemy) to escape of the possible CC he will throw yourself, the combo on the paper looks hard but on the practice its easy. We can use the The Quickness for engage the Grand Entrance, we can active before use Battle Dance or later of hit Grand Entrance for more CC i recommend use before to ensure the hit of Grand Entrance.
Your Grand Entrance can be cancelled if a CC move of cut the move to the center, but if they CC you when you are aready casting the Grand Entrance (Rakan already moved and started the animation of the CC) if the CC dont move you (like a hook), the Grand Entrance will be casted the same way.
you can shield an ally although your animation is interrupted, for a example if veigar put his Event Horizon and you trie to shield to someone from outside this, you will shield him although you got the CC of the Event Horizon, this can be used to save an ally using the Guardian rune, beacuse is activated when an ally you gived a shield or heal take damage.

Rakan can be builded of to much forms, like a tank, a healer and shelder and a combinatoin of the 2, for this you need to play and look for your style and what build is better for you, when you are starting play rakan try with items recomended or meta builds for look how do you play the champ, and when you find your style of play start try items can help to your play style, for a example if your style is be the tank of your team and protect your ADC you will build items like Knight's Vow and Thornmail but all builds have a iteam you will build first and on all games beacuse is the best item for save allys ant is the Locket of the Iron Solari the shield of this iteam can save you or save all your team of be killed before they act.

in this guide i give 2 set of runes for rakan, the classic Guardian with a simple variant, use Precision instead Domination, here i will explicate this change short and simple for later go with aery, a set who depends of the enemy draft:
we will change the Domination set for Precision because we will use the Triumph this rune can save you on fights because rakan sometimes take so much damage, and make you return to help your team and we will use Legend: Haste for reduce the cooldown of the skills of rakan.
For Summon Aery runes i will give a conditions and how to play with this runes:
-first of all this runes are more prepared for play like a healer and shielder, Rakan tank may be less effective with this runes.
This set of runes will be used against draft of poke, this are can have champs like Jinx, Varus, Jhin inter alia. the game style of this set is play shielding and healing the poke damage and for this we will build items like Ardent Censer or Echoes of Helia, making the shields and heals strongest.
For the support item I alaways recommend use Celestial Opposition but you can build Solstice Sleigh for better healing, but play carefully because you will take more damage what if you had builded Celestial Opposition.
on fights with this set you need to spot the allys who need be shielded or healed beacuse you dont have the Guardian with give shield to all allys on the range when is activated.