First of all, all the files on Just A Falling Sim are zipped. Therefore, you will need something that can deal with .rar files. You can get a free piece of software called WinRar which will be able to unzip these files for you. It is found at www.rarlab.com/download.htm. The best one to get is the top one - the graphic and command line one. Even though it is a trial version, once it is over it will keep giving you a message that the trial is over. However, you can just click close, and you can keep on using the software (very nifty!). Instructions for how to use this can be found on its website.
Once you've downloaded them to a folder of your choice (preferably named something like "Sims 2 Downloads", you then need to unzip them into certain folders so that The Sims 2 can find them. This all depends on what it is that you are putting into your game.
Please note that all the instructions that follow assume that you installed The Sims 2 into the default folders. If not, you will have to find the respective folders yourself.
If you are downloading a neighbourhood, then in the zipped file you will find the neighbourhood file, usually something like MyNeighbourhood.sc4, along with 5 images of that neighbourhood. You need to unzip them all to the directory C:\Documents and Settings\yourusername\My Documents\EA Games\The Sims 2\SC4Terrains where "yourusername" is the name you are logged in as on your computer. Make sure that this username is the same one as you intend to play the game on.
Object files will tend to be of the form myobject.package. All objects (including neighbourhood objects) need to be unzipped to C:\Documents and Settings\yourusername\My Documents\EA Games\The Sims 2\Downloads. You can use subfolders to organise your items, for example if we were downloading tables you could have a subdirectory of C:\Documents and Settings\yourusername\My Documents\EA Games\The Sims 2\Downloads\Tables. Then you can extract your table package into this folder. This way you can find objects quicker and is highly recommended.
I will add any more instructions as and when I upload content that doesn't follow the above. Also pay attention to the post that contains the download, as again, I may add what you need to do for specific files.
If you have any questions feel free to email me at sims2@justafallingstar.co.uk (or click the "Contact Admin" link at the bottom of the page) where I will try to help you or at least direct you to a place where you can find a tutorial on the help you need!