Heres a summary:
ALWAYS Pos to Pos Neg to Neg when jumping or charging, it dont matter which battery post happens to be tied to some huge hunk of iron as far as + to + and - to - rule is concerned...
NEVER jump a 12 volt battery direct to a 6, Ive seen them blow up
NEVER make a jump connections last connection near the battery especially around its top where explosive gasses can be present, I usually make the last connection on a good clean heavy frame member AWAY from the battery
If youre using a Neg ground vehicle to jump a Pos ground tractor (still use rule above) dont let the vehicles make contact.
Be safe, 12 vollts really makes her spin