Where do I have to put my downloads?

Depends on the game. First I will explain where you have to put your files for Sims 2.
If the file is .sims2pack, just click twice on it and The Sims 2 Installer will pop up and install it.
If the file is .package, you have to move it into 'Documents\EA GAMES\The Sims 2\Downloads'.

For Sims 3, it works a bit different, and you need to work a bit more with folders.
If the file is .sims3pack, simply click it twice and the game launcher will install it, you can also place it in 'Documents\Electronic Arts\The Sims 3\Downloads' and find the item later in the game launcher, then check it and click install.
If the file is .package, you have to create some folders. There are two ways, one where the folder is located in your Program Files folders, and another that is located in Documents. I preffer Documents one. Go to 'Documents\Electronic Arts\The Sims 3' and create a folder called Mods. Inside that folder, you have to create another one called Packages and you also need a file (Resource.cfg). Once you have both things, just put your .package files into that Package's folder.

How can I open the files?

If the file is .sims2pack, you just have to double click it and the installer will do everything for you.
For .packages (Sims 2) SimPE is your program then. It's the one I use for making the items here. It can also open .sims2pack ones, but I highly recommend you to use the installer if you don't know what you're doing.

For Sims 3, if the file is .sims3pack, double click and it will install. If it's .package (Sims 3) best option is S3PE

All the files uploaded here are compressed with WinRAR, so you will need to download it for uncompressing them. You can also use another programs that can uncompress .rar files.

How can I download your items?
It's very simple. You just have to click in Download and save the file. Usually meshes are included in the main download, so you do not need to download the pack and the mesh, only with the pack is enough.

What are meshes? And, do I have to download them?
Meshes are very important, because is the 3D shape of hairs, clothes... You must download them if you want to see the content in your game. Most of the files have the meshes inside, because I don't like to put them out of the pack, I like everything inside one file, it's easier for everybody.