Thresh: Decisions are hard and it mostly ends up to who has the advantage. If your ADC can follow up (pinging before diving helps), a good rule of thumb would be if you the target or at least get a flash during your CC train, then you should dive (unless it's a teamfight, different rules). I don't play Thresh too often, but when I do I generally just harass with AAs and Flay until they're low or I can get a lantern for my jungler and take them out. I start out yellow gold item, 3 biscuits and a man pot. If they are a harass lane, 4 biscuits. I try not to go back until I have enough money for SS and boots or SS and tier two gold item.
Zyra is squishy. Generally you are going for more hit and run tactics as well as using minions as shields. If you can get some early poke off, you'll enjoy a much better lane because, unless the enemy goes back early, most will be too scared to CS. To get anything out of plants, I found using Q/E then laying a W down (instead of W -> Q/E) works better and faster, giving you more damage. I start blue gold item, 2 biscuits and 2 mana pots unless they are a harass lane, then I get three biscuits. I try not to go back until I have money for SS and boots, or SS and an amplifying tome (or one of the two cheaper upgrades, I like Fiendish Codex or Aether Whisp depending on which item I'm getting first). Don't upgrade blue item, switch it to gold gen line or an AP item later.
Sona works like Zyra, and there's two ways to build her. There's the full AP mid mage way, and the support way. I lean support unless we're going to win. Start lane with 2 biscuits and a 2 mana pots unless it's a harass lane with blue item. If you are going to be assisting the jungle, you want to use your Q once before you leave the fountain and get one stack on your passive. If the jungler starts on the other side of the map, get two stacks of your passive and head to lane. You want your first ideal attack to be AA -> Q -> AA for as much damage, and you want it on whoever will be more annoying (for me, if there's a tank in the lane I try to get the first attack on the ADC so he thinks about following up. If there's a harassing support, they are most likely very squishy and you can hit them with the first attack and hope they don't try to harass as much). Whoever is more active in the lane gets the harass, because they are making the plays, but try to give your attention to both enemies about 60/40. If you are with a premade team and know you are about to snowball out of control, rush Sheen, otherwise, any AP support item works. Switch blue gen item to the yellow one later in the game.
Karma: Play her less than Thresh, won't give advice on something I know almost nothing about.
Soraka. Blue gen item. 2 biscuits 2 mana pots, can opt for 1 biscuit depending on the lane, blue sustain item or yellow, I go blue. Support AP items and some tankiness (not HP, more armor/MR). Leveling should be Q->W->Q->E->Q->R->Max W. Gives you poke in lane that does a fair amount of damage. If you give up an early kill, just max W and AA harass, but never play full passive unless the enemy lane is tons ahead. Switch blue gold item for yellow later.
Nami. Blue gen item, 2 biscuits, 2 mana pots. Harass with W all day every day. If you are below 50% HP, start W on yourself so you get a better heal. Above 75% start on the enemy and stick around for the heal bounce back. In between, go more for the enemy, but be aware that you may lose out. AP support items. Blue gen item switched for yellow gold item later, but you can use the Frost Queens item to land Qs easier.
I buy a lot of potions in lane if I ever have extra money. More sustain means I can harass longer. Get a pink as soon as possible and at least get your tribush, if you are often pushed to the enemies turret, pink the river and green your tribush. Once the enemy starts pushing you back, get a ward in the side bushes. Remind your ADC to ward whenever possible, especially before sightstone. Always get sightstone. Don't ever be set in stone in your build, when you have money for an item, look at the enemy team and your team, identify who is fed and try and counter those champions.
These items rock: 
These are the things I've picked up generally through play. I'm unranked, but play almost nothing but support. These tips are probably not ideal for high plat+, but for learning the role will help.
I have no problems with my Q but I am making wrong decisions which leads me to the death.
I simply die too much because she's squishy as hell and just, I don't know.....
I am simply just spamming my abilities which makes no sense to me :D :D
Its either OK or I get annihilated.
She's kind of hard for me for some reason.
Just YOLO because I make more damage than my ADC and I simply can't Die so I feel comfortable with her.
I suggest trying janna for a bit. She is very forgiving when learning supports, and since she has a knock up, AOE movement buff, and an AD buff shield, you're relevant at all points of the game. Once you get comfy you notice your supporting getting more solid
) because you want to spam your vines and have your spitting flowers ready to go as often as possible. I generally go (in order) 
---> 
, 
, 
might also be a good choice in support as he seems to be more in your style. Swallowing your ADC who's in danger or carrying them for an engage is a god-send.
