Kimura is among the most versatile of submission holds and can be done in a vast array of circumstances. In bottom position however, it can be argued that half guard may be the most practical and effective position to start your training and learning. The angle and distancing, along with the usual circumstances of the situation often make it easier to get to the lock from half guard than it is from more aligned full guard situations. As always - focus on getting the opponents hand quickly behind his back - this will create a huge mechanical and tactical advantage for you that opponents will struggle to deal with.
You can watch tutorial on kimura from top by John right here.