These items are mostly based on Cooldown Reduction.
As Rakan you want to be as fast as you can thats why you need to rush the CDR.
Those items give you the tankiness you need, Some attacking power and extra boostes on your shield for your Attack Damage Carry (ADC).

The support item makes you faster (use the active when you ult to charm fastly)

I have chosen these masteries to gain some extra gold as the support, be more tanky and for the 45% Cooldown Reduction (CDR)

For the runes I made a choice to get some Ability Power (AP) strength.
When you play Rakan all your abilities go on AP.
These runes give you a little boost so you and your adc are stronger and that you are tanky enough to fight.

These order of abilities are so you do enough damage in fights.
Your W is your biggest damage output so you need to max that.
Your Q can heal so it will connect with your item.
Your E is just to jump back and the item activator.
Your R is your most powerfull weapon in game.

The best time is when you have a Xayah in your team.
Xayah makes Rakan better like he can jump further to her.
You can play him with a Xayah but it's not a problem with an other ADC.