There's a lot of libraries that do the hard work for you though... Anyway, if an EKF is too much effort, you could also try the "Madgwick" filter:

Madgwick, S. O., Harrison, A. J., & Vaidyanathan, R. (2011). Estimation of imu and marg orientation using a gradient descent algorithm. In Proceedings of the IEEE International Conference on Rehabilitation Robotics