Everyone has hobbies; some people collect trading cards, some build models, some grow plants .. I, myself, prefer to grow sadistic Doms. Over the last few days I've been fortunate enough to have a random, unexpected in-surge of curious newcomers. When dealing with Dom-curious guys, one of the most common queries presented during scene negotiations comes after we discuss a bondage position that suits their capabilities: "Okay, then what?" Due to the rapid succession, I realized the conversation from there goes pretty similarly so I figured inexperienced, Dom-aspiring readers - or better yet, anyone looking to take on my hobby - may find a more general overview to be useful.
So what next?
Whether or not you intend on pursuing a sadistic side, it's important to figure out what gets you physically hard or mentally pleased. When it comes to sadism, the strongest satisfaction usually comes from the recipient's reactions as opposed to your own actions, in that an action obviously does not indicate the recipient's mindset. Without this input, I'd argue it can't even properly be regarded as sadism. The only real way to find what pleases you is to experiment - if you've jerked off to any bondage porn, odds are you have at least a rough idea. Barring a labeled, exhaustive list of every fetish, I can only go over a few activities that are more likely to elicit given reactions. Much of this may seem self-apparent at a glance, but it's logically deconstructing separate reactions and levels of intensity that don't often occur to people naturally. Even if you have some experience as a sadist, if you don't quite feel you've found your niche it may still be a good exercise.
Disclaimer: the text below assumes a proper scene negotiation has occurred, discussing limits and expectations at length. If a sub has not expressed they wish to explore masochism, they are not the sub to test these ideas on.
The simplest and easiest reaction to pursue is directly pleasing a boy - most notably getting him hard. This can be as common as edging, or as uncommon as a specific fetish/trigger like ball-crushing. It's sometimes a little difficult for non-sadistic tops to play with masochistic bottoms, given that what they directly enjoy is not aligned. If you're uncomfortable administering pain, just gauge how hard the boy is and whether he's getting harder or softer in response to any sort of pain. Bringing this to a sadistic level, just show him you can give him what he wants to an overwhelming degree and then take it away from him abruptly; in no time he'll be begging for it - perhaps literally - all without ever actually doing anything unpleasant.
If you've gotten comfortable with that, the easiest way to start moving away from directly pleasing a sub is to make his body want to respond reflexively. Since there's usually not much of a mental component to muscle reflex, it's easier to test the waters without jeopardizing any headspace the boy may be in. Wax, short/fast bursts of cbt, tickling, certain electro not attached to the sub (cattle prod, violet wand, etc) and most things sharp and sudden are great ways to get a guy's body to fight unintentionally. A blindfold greatly assists here, so a boy can't brace himself to control his reflexes. Predicament bondage is also a great way to slowly get a sub to struggle somewhat, given the constant need to readjust - not quite the same as a sudden jolt, but an unconscious sort of movement. This may sound more advanced, but anyone with some rope and an overhead hook/pipe/eyebolt should be able to work out something safe and simple, like a boy on his back with his wrists elevated by rope tied to his balls.
From here it's easy to progress to something like grunting or instinctual verbal protest. Prolonged nipple play, slow cbt (vices, IcyHot, clothespins, etc), or TENS unit-like electro boxes are great for this. Anything that builds intensity linearly is a prime choice, since you can control intensity in a more calculated fashion. After a while predicament bondage can also work for this, as he transitions from naturally readjusting/struggling for sustained comfort to actively needing to choose between pain/discomfort in one place or pain in another. At this point we're still eliciting something rooted mostly in reflexive responses, but we're starting to bring the boy's head into it.
By this point if you still derive enjoyment from tormenting the boy, you can try pushing him to a state of willful resistance. Virtually anything is suitable for this, it's just a matter of finding out what a sub is frustrated by or actively dislikes. For instance, while I enjoy some cbt I truly hate having my cockhead slapped; while it's not a limit, it's a quick way to frustrate me since I can't control my body's reaction and my brain thinks and hopes every slap will be the last. This may be a specific act or more likely a level of intensity. Either way, most subs who are subjected to something they dislike or are frustrated by will eventually become obstinate and willful. Anything that causes a sub to attempt to mitigate his reaction is probably a good start, since he doesn't want you to notice how it substantially it affects him. At this point there's no question: the boy's mind is fully engaged in some form of dislike for what is occurring, which is unquestionably sadistic.
These reactions should be adequate to serve as a means to show you more specifically where you should focus to move forward. Even if you don't directly enjoy a particular reaction, experiencing it can help you sort out what dynamics you might be suited for. If you're not at all interested in directly pleasing a boy, you might not find much use for bondage bottoms who do not engage in D/s dynamics since they will likely expect it. If you don't enjoy making a boy willfully resist but were comfortable taking him there, then you might enjoy a rules-oriented D/s relationship that utilizes corporal punishment. Whatever this initial experimentation teaches you, it should put enough in your toolbox to start building the dynamics that will allow you explore further.