Draw a finite state machine for a welding machine
The flip flop is used for change states over the time using a clock. If you go on wikipwdia you can easily find Flip Flop D description.

You can read this link for learning FSM: 

http://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Seq/fsm.html

First you learn sequential logic you have to know basic combinatory logic rules. If you don't I reccomend to read something about it.

You can practice using "logicly" software, search it on google. Have fun!