You can push-start a manual transmission car alone (with no-one to help you push) if you can push in reverse (using your back) to back the car up onto a slight hill. Not too steep otherwise you will not be able to push it up. Long and almost flat is the best. Even up a moderately steep hill, you can push car up one step at a time by your leg and back muscles.
Keeping the driver's door opened, put transmission into first gear, push the clutch down with your right leg, and push car backwards uphill with your left leg through the opened door (assuming steering wheel is on the left side of the car). When you are tired, disengage clutch and car will stop in the middle of the hill. Repeat until you are far enough uphill so that you have chance to develop enough speed downhill.
Then, to start, you put the car into neutral, push the car forward a bit, jump in, wait for a bit speed to develop using gravity, and put it into first gear to start engine (and pray it works first time so you don't have to repeat).
If you are in flat area with no hill to back up to, starting in reverse gives you more power: Get into reverse, push clutch with right leg and push thru opened door with left leg to develop some speed, then release clutch to try start engine. Be prepared to press clutch again, because when car starts, will have not enough speed to run (and might die like when you slow down without engaging the clutch).
If your batteries are dead because you forgot to turn off lights (for few hours not days, so battery is not 100% dead, only nearly dead and has not enough juice to start your car): Before starting, turn off the lights and wait an hour or few to let the battery recover. You may also use time to find someone to jump-start car or help you to push :-)