Ok, all you need is three things. First of all, a POL fitting which screws into the threads on the inside of the valve on your 100 lbs tank. Next is a section (however long you need it) of HIGH pressure LP hose. Third is the male end of a ACME connection. Using yellow PVC tape (not white it is for water) wrap the connections and tighten them so you don't have any leaks. The POL fitting goes into the valve on the 100 lbs cylinder and the male end of the ACME connection goes into one of the green connectors on your regulator pigtail. Leave the other tank connected. Set the regulator to point at the connection with the 100 lbs tank. The regulator will switch over when the 100 lbs tank is empty. Send me a PM and I will send you some pictures and suppliers.
