Please help with 3 way dimmer hookup with existing 2 way switches
This isn't so hard once you figure out what's what

From the instructions you posted, we know that the black wire on your new dimmer is  the common, and the two red wires on the dimmer are the two travelers.  Coming from the wall, we can tell that the black wire that is spliced to the wire going off to the single pole switch is the always-hot feed coming in from the fact it's connected to that single pole switch, as well as another incoming wire.  This means that we can safely hook that up to the black wire on the dimmer, and also connect the green wire on the dimmer to the grounding bundle in the box if present or land it on a ground screw in the back of the box if there's no grounds present, then connect the remaining open wires from the wall to the red wires from the dimmer since we know they're the travelers by process of elimination.