This is fairly easy: Your forklift battery bank will consist of 18 x 2V cells connected in series. Usually it will have three rows of six cells/row. Each row is a 12V battery. You will have to disconnect the links after every six cells which will leave you with three separate 12V banks. It is simply a matter of connecting the three 12V banks in parallel for a high capacity 12V battery, i.e. connect the three positive poles together then the three negative poles. (For the interconnections between the different 12V battery banks, you might find that you'll have to use longer jumpers; just make sure the cables are thick enough!)
Good luck and I hope all the cells are still usable, otherwise you'll have to sort them out and put 2 banks of 12V in parallel.