I would add a gallon or 2 of bleach to the tank, fill it up and run the filters.And let it run for at least 24 hours.
Anything organic in the tank will turn white (such any algae left on the side, etc).
Drain the tank, fill it up again, and add dechorinator appropriate for the number of gallons.
Run another day, then add ammonia to begin a new "fish less" cycle.
Or not adding ammonia, get some cheap sacrificial fish for a "fish in" cycle
Bottled bacteria could save some time, in a week or 2 I'd start testing he water for ammonia, nitrite, and nitrate .
A normal cycle would take 6 to 8 weeks.
I would not add any "important fish until the tank is totally cycled (no ammonia, no nitrite, some nitrate)