Alright, so we are just gonna briefly outline the necessities for weather and making a weather team.
Step #1: You must have weather (duh). All the weather inducers are as listed...
Hail: Snover (RU), Abomasnow (UU)
Sand: Hippopotas (Sand is only allowed in OU +, so it's better to use his evolution), Hippodown (OU), Tyranitar (OU)
Rain: Politoed (OU), Kyogre (Uber)
Sun: Ninetales (OU), Groudon (Uber)
Step #2: Provide abusers of your weather.
So basically, you want to abuse abilities like Chlorophyll (if using Sun), or Sand Rush (if using Sand). Pokemons with abilities like those are usually very important and a crucial part to your team (though not always).
Another thing would be to use weather to your advantage. The most common examples would be Rain and Sand. Rain provides a decrese in Fire attacks, so it's safe to use Steel types such as Ferrothorn, Skarmory, and such. Sand gives a 1.5 boost to a Rock type's SDef. So Tyranitar not only sets up sand, but gets a boost from it as well. Terrakion is also pretty well seen in sand, and survives a bit longer, thanks to the boost. Things like these are the things you should be looking for.
Step #3: Get the Synergy right.
This part is not always for weather teams, but for all teams in general. The purpose is to find Pokemons that cover your weather inducer very well. (Ex: Ferrothorn covers Politoed pretty well) Not only does this allow safer switch-in's, but let's you keep the momentum, if you were ever were to encounter yourself in a Weather War (a battle where you and your opponent have weather, i.e Sun vs Rain), and happens pretty often, as OU is pretty much the weather battleground. This leads to cores, and I'm gonna let you find some on your own :P
Step #4: Have a counter.
No, I don't mean that you must have a Pokemon with Counter. What I mean is, have a Pokemon that can fight against other weather, just in case your weather inducer faints in battle before the opponent's does. So Volcarona in Rain may seem pretty odd, but note the fact that is can abuse Sun, take Hail down, and put a big dent in Sand. There are counters to use in every weather team, and your job is to find them. I'm just gonna list some, and you find out where they belong (No, I will not do it for you :P)
(Notable) Weather Counters:
Kingdra
Volcarona
Rotom-W
Landorus/Landorus-Therian
Breloom
Reuniclus
Alakazam
Terrakion
And much much more...
Step #5a: Make sure the team synergizes (covers each other) pretty well.
Well, this is mainly the clean-up of the team. Like any other team, make sure that your team is balanced out, and that you don't have a a large, or noticable weakness. If so, it is usually best to go back and see if anything else could fit in and do a better job. Test the team out in a couple of matches to find best results.
Step #5b: Enjoy the work of art you have made, and use it with pride :D
