Here is some documentation from the KJ-3100 manual. This should be the same for any jetter in a situation where there is inadequate water supply.
In cases of insufficient water flow, possible solutions include using fittings to allow multiple supply hoses to be connected to the jetter or the use of a tank.
If a tank is used, plumb a tee with full port valves to the jetter water inlet as shown in Figure 9. Connect a
3/4" (19 mm) hose no longer than 6 feet (1.8 m) to the valve on the outlet of the tee, and connect the water supply to the run of the tee. Either place the hose end in the tank or connect to the tank outlet. The entire length of the tank hose should be no more than 5" (12.7 cm) above the jetter water inlet, or the jetter will not draw water from the tank.
Fill the tank prior to starting the jetter. When starting jetter, turn the tank valve off. As soon as the jetter is started, open the tank valve. Monitor the tank water level, and if needed, stop jetting to allow the tank to refill. Do not allow the water level to fall below the hose end.