First start the game on free-roaming
When started head over to this place in the map

go to the corner of that place and u sould see a place with sand

go up and a bit right of the sand place and u sould see the pipe just right there

now simpely jump in and you will be warped to the minecraft world and you will get a little cutscene

and then tada you unlocked steve

i hope this guide has been helpfull for you.