The best advice I have been given when learning the speed bag was on day 1-3, don't expect to get a rhythm going, just get your hand and eyes use to the movement, so go slow, your brain will just begin to connect the dots of you keep doing this.
Day 4-6 you'll get to the point where you'll start picking up a bit of speed on the speed bag as your hand eye coordination begins to map itself together.
Day 7 - x you'll have a base line competencie when it comes to hitting the speed bag, then you can start building this into your MMA training routine, you wont be disappointed! It is well worth the effort.
Checkout the above YouTube video from Coach Anthony Boxing. It helps explain the basics but don't forget that the first few days is more about neurology than your hands.