So i've have some Blue LED's leftover and i want to use them to make some interior footwell lights that come on when the car is started. How do i go about doing this? Anyone done something similar?

Yes, the physical slots in the underside of the dash are already there.

You just need to place the globe in the globe holder into the slot and wire them to the interior roof light circuit. The best place the top into it is the drivers side A Pillar.

Depending on the model car you have will depend if everything is there or not. i got a TF magna and had to get the foot well light covers, T10 bulb holders and wiring, at the wreckers you could probably get the lot for about $10, get the light covers and bulb holders with wire attached from a second gen magna. i tapped in the the interior light harness from the drivers side pillar, just pull off the plastic trim