Easy, But doing it well takes a bit of thought. You are going to have to get wires to them and hide them. Plan how, in my expereince and see if someone can show you a good route - under edges of carpet and trim for example. Where will you dril lthe body? This needs to be sealed and few 1/16ths either way could be the differnce between a nice hole and a broken drill struck in a flange you didn't know was there.
Decide wether you want it:
1 able to be turned on anytime without the key
2 only with the keys in.
3 only with the lights on
1 is most useful, 2 stops you leaving them on and getting flat battery, 3 allows a bodge as you may (MAY) be able to piggly back low current LEDs off a rear light (canbus excepted).