Answer: You don’t. Since PAR and PAR2 files come in sets, at first glance, it looks like you need to join them, but you don’t. PAR2 files are designed this way to reduce how much data that you need to download from Usenet.
PAR2 files contain error-correction data. So, if you download some RAR files, and they are OK, then you don’t need to download the associated PARs. If your RAR files have some minor damage, then you might only need to download one PAR2 file to fix them. If your RARs have a lot of damage, or one of the RAR files is missing completely, then you might have to download all of the PAR files.
You know that you need the PAR files because when you tried to extract the RAR files, WinRAR gave you an error. You know how many PAR “blocks” you need because QuickPAR will tell you exactly how much damage your RAR files have. Once you know the damage, then you can decide which PAR files to download.
See our PAR2 files page for more info.