as for the plugs, have you checked their Gap? if that is fucked, they won't work.
Also, make sure the wires are actually snapping down and making a good connection. Sounds silly, but sometimes they just don't like going over new plugs.

And there is a big big big big difference between back firing and misfiring.
(just like there is a big difference between turning the key and getting a click and turning the key and nothing happening)
Be clear with what you are saying and you can get much more clear answers.

If it is misfiring, then it could be the two things that I started.
if is it back firing, then you have a timing problem and you've probably been running on two cylinders for a while and didn't even know it and now that you put actual working spark plugs in it, it has become a problem.

What rhart said is a very good possibility too.

Hell, it is very possible that you just made a mistake and put the wrong wire down the wrong whole. Getting number 1 and number 2 mixed up will cause a back fire.
The more I think about it, the more I am leaning towards that being the issue seeing as how you seem to have little experience working on cars and probably pulled all of the wires out at once. Slow down. Do things one at a time. Look things over before you start the engine. Develop good habits while you are learning doing the little things so when it comes time to do something important, you don't screw it up because you had to test it by starting it. Think things through. if it doesn't make sense, then stop, go back and either make it make sense, or asking a clear, specific question.
A KA is a very very very easy engine to learn on. Everything makes sense. Just think it through.