what you need:
good eyes
reflexes
somebody else who wants to play RPS irl
step 1:
find somebody to engage in rock, paper, scissors with you oh my
step 2:
start playing rock, paper scissors
step 3:
choose rock
step 4:
watch closely at their hand when they say 'scissors' or '3' or whatever you say last
chances are you can see what they picked before they finish shaking their arm
step 5:
quickly change from rock to whatever will beat them
rock -> rock
rock -> paper
rock -> scissors
rock -> gun
etc.
step 6:
tell them youre psychic
now no one will even guess that youre cheating!
what else to know:
dont play with somebody who also does this
be able to change it on reflex, dont think about it too much