(1) You need a 24V supply to boost it - i.e. another 24 V lorry or set of batteries.
(2) There will be 2 No 12V batteries. The negative of battery A will be connected by a link to the positive of battery B. The negative of Battery B will be attached to the chassis (negative ground).
So starting at the flat battery, connect the positive of (flat) battery A to the positive of the booster Battery A and the negative lead from the booster battery B to the negative of flat battery B or ideally to a a good earth on the chassis away from the flat battery.
Make sure the 2 lorries aren't in contact.
Also, stating the obvious: you need extra long jump leads - standard leads for a car won't be long enough - especially if the vehicles can't be positioned with the battery carriers opposite each other - the jump leads may have to pass over, under or through the body/chassis.
JD
December 2011