I’m making a trick in which i end up with some slack before reaching the next step, i was wondering what i could do with that slack before continuing the trick to make it look nicer…? any tips on how to use slack or any video recommendations on how to use slack?
Well that really depends on what trick your getting into that will create that slack and what trick you plan on using after the slack. Maybe throwing it around the string connected to yoyo and grabbing it with your throw hand or making a loop and landing the yoyo on it or through it. Lots to do but that depends on what tricks are implied.
Could you show us a video of your Yoyo trick so far so we could give you ideas?
Would need to see what kind of slack it is
How does one upload pictures/ videos to this?
One uploads them to YouTube, Vimeo, or similar service and then shares the link here. 
Quick and dirty if you can pull it off in 15 seconds, I’ve been using Instagram. But yea, Youtube, Vimeo, etc…
I’ve been currently playing around with different tricks/combos with the tricks I do know from the advanced, trying to pull myself out of the “Double or Nothing 1.5 mount Kwiji-Matrix combo” that tends to happen at this point in the yoyo timeline, starting to play with slack, enjoying Cheese Whip, which gets you into a wrist mount quickly.