How's it? My name is Justin and I'm a Zyra main. I'm not exactly high ELO, but I play with mostly Plat and Diamond level players, so I have solid experience. This guide is geared towards a bot-lane support that has the option to carry in the late game.

Zyra is a mage/support champ that has great CC and damage but is quite squishy and immobile. She is a playmaking support that can burst down an enemy quite quickly, allowing the ADC to finish off the kill.

Pros
Great CC
High kill pressure
Auto attack range is longer than majority of enemy ADC/Supports
Can engage/disengage
Seeds and plants can give vision control advantage
Great zoning

Cons
Quite immobile
Squishy
No sustain
Reliant on landing skills

Zyra is an admittedly difficult champion to play initially, but her major combo is easy enough to learn with a bit of practice.

Major Combo: E>W>Q>W>R

Because Zyra's skills have a slightly delayed cast time, it is a good idea to place your seeds after the other skills have been cast. Sometimes the enemy can dodge your skils if you place the seeds first. The timing of the combo is what makes Zyra hard to play. However, if you can land her snare and activate a melee plant, and then Q the enemy and activate a ranged plant, then that can lead to a large amount of damage. Add in her ult, and you can deal enough CC and damage for your ADC to mop up.

Zyra is quite useful if she can get max out her CDR. Morellonomicon is a really solid first buy because of decent AP and CDR, plus Zyra's plants apply the Grievous Wounds. Liandry's Torment is also a decent item since it gives ok AP and Health. Plants also apply the spell damage over time, acting as a mini-ignite. I personally like Morellonomicon but Liandry's is useful if you need more health in the laning phase. I prefer Boots of Mobility so that Zyra can be a bit faster when roaming to place wards or support other lanes.

I usually go for damage items unless my ADC needs a Mikhael's to survive in lane. Zhonya's is nice because Zyra's plants still attack even if she is frozen in stasis. It's a little unusual, but I like Banner of Command because it gives AP, CDR and the option to help split push a little bit.

Zyra also gets strong in the late game, and can possibly carry at full build, so building items like Zhonya's and Deathcap can give you enough AP to solo kill an ADC or mid-lane assassin/fighter.

The traditional 0/9/21 is normal on most supports, with the 9 giving you a little more tankiness, while the 21 in utility helps in other areas. You can get a few points in offense if you need them, but those are quite situational.

I usually go with AP Quints, Armor Seals, MR Glyphs, and Magic Pen Marks. This helps give a little bit more defense in the early levels, while you have a bit more kill pressure early on with the AP and Magic Pen.

During the majority of the game, Zyra is most useful for her CC. The best way to help out is to constantly peel for your team and try to land snares so that your team can blow up the enemy champs. Also remember to always put down plants when casting skills so that your plants do damage as well.

In the late game, unless you built all defensive items or the enemy team is pretty fed, there is a good chance you can combo down the ADC or other champs with someone else's damage. Getting vision and camping bushes can pay off if the other team tends to roam around by themselves.