Same way you would make an un-unchained...or rather chained rogue.
Not sure exactly what you are looking for. Unchained is not much better than before with regards to ranged sneak attack. If anything, they are worse since you don't get dex to damage benefit that the melee unchained do.
But if you go human for the extra feat, or spend combat trick you'll have the basic gamut of point-blank, precise and rapid shot feats by 3rd.
Are you restricted on traits or archetypes? Because an archeologist bard, urban ranger, seeker sorcerer would be strong if not superior options other than a rogue.
Or have a character get Trap Finder...and get the rogue class feature for the cost of a trait.
FYI, I've been playing an unchained rogue and while they are better than before they are still a class that is not needed in a party.
