You might also want to consider asking other sub service bottoms for advice. Many of us have already been trained and serviced doms and can provide some insights about how best to service a top.
As someone who served an alpha top for close to two years, I can say that one of the main attributes of being a good sub bottom is letting go of your sense of self, your sense of agency, and free will. While a good dom will set boundaries, there should only be a few and anything inside those boundaries is fair game. You have to understand that you don't have preferences. You cannot express preferences about things you will or won't do or how you like to do things. You have to understand that when servicing a dom, it's not a negotiation; it's obedience. You have to understand that you're not an agent of yourself, but an object of service to be used as and when the top wants.
You also have to let go of any sense of "masculinity". Most subs are fully shaved and many are caged. Your penis has no role. I was caged because my alpha top did not even want to see my penis. To him, it is a clit, my hole is a pussy. My instructions were to always be shaved, caged, and plugged. Whenever I came to his place, I was to strip down immediately without waiting to be told. This is done on purpose; there is only one man when you are around your dom and that is Him.
You have to get comfortable with doing things that are in your best interest, but you might not be aware of how those things are slowly and quietly changing and shaping your mindset -- your view of Him and of yourself. You have to get comfortable with being told to sleep on the floor while he plays with another sub in the bed. You're rendered meaningless. Some other sub is getting the dick that you thought was yours, feeling jealous while listening to all those moans, wondering whether this new sub is going to push you out of your connection with your dom.
As you're lying on the floor, wondering whether the dom has found a new toy and your time with him is over, you hear one word "cleanup" and you immediately understand that you're being told to clean them both up after their fuck. You climb onto the bed and see your dom's now soft dick wet with ass juice and cum and start to clean it up. Then your dom tells the other fag, "stick your ass up, it's going to clean you". You turn to the fag and rim and felch him clean.
You have to get comfortable being told that the only way that you can piss is outside, on all 4's, with one leg raised. You're wondering what happens when it rains outside, and he has already anticipated that and tells you you can go inside on a pee pad, but still on all 4's with one leg raised -- like a lowly animal.
You train to be a urinal so that whenever your dom needs to piss, he does not have to be inconvenienced with needing to get up and go to the bathroom. He can use your mouth right there.
You have to get comfortable taking the trash to the curb fully naked, for anyone around to see.
Thought you'd like to know....