Hi, I'm a Zyra with more than 1.000.000 mastery points.
This guide is useful to reach Diamond 4, but it is not good in higher elo.
The goal of this guide is to snowball as much as possible to solo carry the game and end quickly.

Zyra is the best champion in LoL for leashing your jungler. Make sure to get out of the bush at 00:59 so your passive works 3 times before 1:30. ( in low elo, don't hesitate to tell your jungler to let the plants tank).
Given that you pull is fast and great, you can usually get in lane before the enemy duo. Your goal now is to take level 2 before the enemy and get the first blood.
If you succeed to get you level 2 before them, you will be able to have a 1st blood of a flash in 80% of the case at this elo.
If you didn't managed to kill at level 2, you can try to keep the wave in the middle of the lane, get the level 3 before enemies and try again.
If you start pushing, make sure you ward the river to avoid gank and help your jungler with the scuttle crab ( you can even do it with your adc if both junglers are topside).

After getting your level 3, taking 1 kill or assist, or just a huge advantage in lane, you should recall.
With more than 575 gold, you should buy Refillable, Dark Seal and a pink ward.

The goal now is to take kills and plates.
With refillable and biscuits, you will be able to harass the enemy and heal yourself. Dark Seal give you some AP and health, and helps you to take the advantage in your lane. At this point, you should be able to take some kills with E-W-Q-W combo ( +ignite).
Don't be scared of flashing next to the enemies to surprise them and to do your combo, your early kills are more valuable than most of others champions.
-> If you are losing the lane, heal yourself with refillable and biscuits and wait your level 6.

Take 1 or 2 reset between min 7 and 9 in order to have more AP.
You need to make sure that you will be able to buy you boots as soon as possible, which means before min 11.

The goal now is to take 1st turret and as much plates as possible.
At this point of the game, you should have upgraded boots, dark seal and maybe codex.

Your goal now is to take as much plates as possible before 14:00. If you have taken the first turret, you should consider doing the Herald. Indeed you can totally solo it, but you should ask help from your jungler/ Mid.
If you can't do it, go mid for a gank and then help push the turret.
At this point, you should be able to buy Liandry.

You goal now is to play safe and wait for Zhonya to make crazy moves.
At this point of the game, you will probably group with your team frequently and what you should do is too situational to be explained in detail in this guide.
However, you should be careful: your dark seal is probably stacked and you have a lot of damage. Yet, you are still very squishy and it is at this point of the game where you SHOULD NOT DIE. If you manage to get your Zhonya without dying too much, then you will probably win the game.

After your Zhonya, don't hesitate to do your full combo by flanking the enemy and do some Flash+E+W+W+Q+R+Ignite+Zhonya.
If you do it correctly, you can win the game alone.