for an older engine, it might do harm than not

when you do flushing, the sludge will come into circulation before you pour it out. during the process it could potentially clog the oil pump starving it to death.
with today's oil quality and technology, there is a very slim necessity to flush a gas engine unless you are sure it's absolutely needed. if you are not sure, just use synthetic oil next time it will dissolve sludge as well but in a mild manner.

if you do decide to do it, there is a product in most auto parts stores, it's called "engine flush" or "engine restore" in a metal can.  right before changing oil, pour it in and run engine for 5-10 minutes, idle only, don't drive the car.  when done, drain it competely, change filter and refill oil.  it's simple but powerful.