Most small bikes are relatively easy to bump start. Big twin-cylinder engines are almost impossible and anything with a slipper clutch (post-2005 sports bikes mostly) are physically impossible, so don’t bother.
To bump start a bike, find a straight, clear stretch of road and put the bike into second gear. Make sure the ignition, choke and fuel are on and with the clutch pulled in, push the bike along until you get to jogging pace. Then, release the clutch. You will feel the drag of the engine turning over and hopefully, it will fire shortly afterwards. As soon as it fires, pull the clutch in, stop the bike and either rev the engine or let it rev on the choke.
Don’t attempt to ride off until it is warm enough to idle smoothly and don’t switch it off until you have ridden for at least 20 minutes or chances are it won’t start again if you let it cool down.
Bigger engines may need the help of a friend. Sit on the bike in neutral and get them to push you along. When you have a reasonable speed up, put the bike into second and as you release the clutch bounce onto the seat to put extra weight on the back wheel.