sensor fusion on Android (accelerometer + GPS) using Extended Kalman Filter to estimate speed

I'm trying to understand the process of sensor fusion and extended kalman filtering. My objective is to estimate the speed of a moving car using accelerometer and GPS measurements. I'm obliged to use the Extended Kalman Filter but I've never used it before and I didn't find much about what I'm looking for on the internet. I'm looking for some orientation here. P.S: I wanted to use the Kalman Filter of OpenCV but I don't know how to modify it to get the extended version