I have a popcorn maker that when you plug it in it just starts working. Theres no on/off switch and it's kind of annoying. When you unplug it you always get a big blue spark which seems kind of dangerous to me, it might not be but it seems like it would be. It has a standard 2 prong AC power cable. How can I make an on/off switch for this without breaking it or shocking myself?
The easiest and safest method is to buy some form of switched outlet with a cord and plug. It would be possible to make one yourself but the cost will exceed that of a premade one and there are significant safety issues.
The spark on disconnection is not dangerous as long as there is not flammable gas present - in which case the danger would be extreme even without the spark.
As long as you stay out of contact with the mains connections there should be no great danger from the equipment.
Plug in switches like the one shown below are available from here. Allowable current not specified.
You can IR remote control appliances up to 10A current rating with items like this one from here for $US6.