Unzip the zip file into a temporary folder. 

If you want the set to replace the default pajamas, replace the set name with 01.  
For example:  PajamaFLgt_MosaicPurple.bmp would become PajamaFLgt_01.bmp
Then put the files into the Maxis/TheSims/GameData/Skins folder.  

If you want the set to just work with a particular outfit:  Find the outfit in your Maxis/TheSims/GameData/Skins folder, copy the outfit name, and replace the set name with the outfit name from the Skins folder.

Example:

Outfit bmp (from Skins folder):  B001FCChdlgt_pinkshortalls.bmp
Set files: PajamaFLgt_PastelPurple.bmp, UFChdLgt_PastelPurple.bmp
Change names to: PajamaFLgt_pinkshortalls.bmp, UFChdLgt_pinkshortalls.bmp

The pajamas will start working automatically, but for the new swimsuits to show up you need to have your sim change clothes until they're back in their old outfit.  Sims that are created with the outfit after you install the swimsuit will use the new swimsuit.

If you have problems, contact me at joy_lyn@hotmail.com

~joy