Fetlife used to be a great place but has since been taken over by TNG’s and fifty shaders so has become more and more of a soft porn website. A lot of the kink has ended up being blacklisted.
D/s is a relationship dynamic and needs to be fulfilling for both people. It’s not something you “do” it’s how you do your relationship or engage sexually so it’s up to the two people in the relationship to determine the ‘rules’ of engagement, so to speak.
It’s a well worn cliche but communication really is key.
First off, what’s a hard limit. What are the things either of you are simply not willing to do? For some people that’s lasting marks for example, for others it’s far more along the SM scale. Find your level, somewhere you are both comfortable. That doesn’t mean you can’t re-evaluate and explore further in the future but at least you start with a clear playing field.
Is there something you and your sub can do to signal the start of playtime, if it’s not a 24/7 thing. A little ritual, that is meaningful to you both, the sub many enjoy kneeling quietly for a short time or fetching and laying out toys etc or it could be something a bit more personal.
Safe words/traffic lights... always good practice, unless you’re getting into heavier consensual non consent (CNC) which it doesn’t sound like you are.
Spanking. Slapping an arse with your hand isn’t all that dangerous in the grand scheme of things. Back of thighs hurt more, make sure the sub is comfortably positioned (so hip bones don’t dig into hard things like a table edge for example) and stick to the fleshy bits. If it’s OTK, there’s a chance they may get a dizzy head rush if they’re there too long but you’ll be checking in regularly anyway. Making someone count the spanks can be fun :) they get their numbers mixed up, back to one we go. Also helps you hear tone of voice, where they are at in terms of close to being done. You could also get your sub to thank you and ask for another after each one or ten. Encouraging feedback is a good thing to do.
Adding in implements and you need much more care - and practice first, preferably not on a human - pillows are very forgiving of mistakes, subs not so much. If you are using something on someone else, always try it on yourself first. Your calf is a good measure of the force and strength of ouch you’re likely to inflict; palm of your hand not so much until you have a bit more experience.
afterwards, know what your sub needs in advance of even starting and allow for unexpected emotions.
Be safe & Have fun!