I'm not sure on a guide for how to do that, but I just want to throw this out there for you to think about before going ahead and doing this.
If you are using the internal HDD that Mac OS X is running on as the TM drive, then you:
a) may notice big performance decreases as the TM backs up data.
b) will run out of space on the internal HDD really quick
c) are completely hooped if that drive fails. you have nothing to recover from then.
If you're still keen on doing this, then I would first advise that you read through point 'c' a couple more times.. just to let it sink in 🙂
In all seriousness, if you use disk utility to partition the drive, then you should be able to use it for your TM backups.