Ok, so this will be pretty hard... anyway so, I'm just gonna tell you what I'd do.
So first I'd get a copy of the default Minecraft resource pack. Then when you go into the folder just change the textures and such.
Some warnings:
One, Being that some blocks such as grass use special textures, Just drawing all over it will look odd and not look how you want to all the time. (depends on what colour you want it.)
Two, tying to make costume sounds will be very hard. Since 1.7 they changed the way sounds work. However there are ways to fix this, there are some programs that will do this for you. But that is known be a little hard!
Three, When you have finished the resource pack remember to put it into %appdata%/.minecraft/resourcepacks then select it in game. and start game and test it out!
Oh and if you look hard enough you will find every thing you can change, E.g the image that comes up when you put on a pumpkin, or the look of a enchanted item. (the purple flashy bits.)
Hope this helps somewhat, But if it didn't then... i have my excuses, My first post :3
Oh and i was just tying to say everything that might help. since as i don't know how good you are at it at all so i just acted like you needed to know!
In the language folder where your en_US.lang file is, delete the en_GB.lang, en_AU.lang and en_CA.lang (If they exist). Modify the names in the en_US.lang file. Now copy and paste the modified en_US.lang file 3 times and rename the files en_GB.lang, en_AU.lang and en_CA.lang. If you modify the en_US.lang file again you will have to repeat this process.
Best Answer
For Windows
Are you using Windows?
Put everything back how it was, then just change the textures in the Weather.png file to snowflakes. If you are not using Windows or need any additional help, just reply back.