Bill1234 and MagnaNasakki gave some good advice - I would like to add a little just for some clarity sake. Don't just keep your foot outside his right, keep it NEXT TO and outside his right. Close on the tall southpaw all the time from off angle. The great advantage a lefty has is he fights against orthodox fighters almost all the time and he is used to it, the orthodox fighter sees southpaws rarely and southpaws are strange and backward to most of the things the orthodox fighter trains. The great DIS-advantage of the southpaw stance is that he LEADS with his liver. You should be hooking to his head from outside his right foot and crossing to his liver then take another half step to your left and hook to his liver and cross "down the pipe" to his chin. His right hand WILL come down when you pound the liver and then he isn't a southpaw, he's meat...
Fine advice.I'd also not forget to state they are best delivered while relentlessly marching forward in a straight line.