Tegrak lagfix: how do I install it? http://forum.xda-developers.com/showthread.php?t=920711

I've seen the above thread but I have no idea how to install the kernel. I've got a Galaxy S running Froyo 2.2 "JPO". 

Since removing the OCLF lagfix the phone has been really slow, so I need to find out what I need to do to get Tegrak working. 

I have Odin 1.3 which I used to install Froyo, but that was following a detailed step by step list of instructions!

It says in the thread I need the latest stock firmware before trying to install Tegrak, but it doesn't state what that is or where to get it. It also doesn't really go into detail beyond saying "use Odin to install it".

Additionally, how do I go about making sure the apps I have bought from the market are saved so I don't have to buy them again?

I'm a web developer, so it's not as if the tech stuff is alien to me, but the Android specific stuff is!
Load the kernel .zip file into the PDA field of Odin, and (if you have it, should have come with Odin) load the 512 .pit file into the PIT field of Odin.  Then flash it just like you did for the Froyo ROM.

A stock ROM is just a ROM that's not modified by anyone, it came straight from Samsung.  If you aren't using a lagfix already, you probably don't need a stock ROM however.  You should be able to find one easily enough in the XDA subforum for your device, if you want it; use JS5, since that's what the kernel is based on.

Your paid apps are recorded in your Google account.  You will never lose your purchase rights to them (barring any service suspensions and that sort of thing).  You can, however, use something like Titanium backup to back them up (requires root).  You can back up the entire device using ClockworkMod / Nandroid; look for ROM Manager in the Market (requires root).

One question: Why are you using that over straight Voodoo?  Voodoo's perfect, and I believe Supercurio has taken stock kernels and added only Voodoo so you don't get any other random stuff.