They have to gather signatures for petitions, which wouldn't be a problem for Trump from a numbers standpoint; but having to develop teams to navigate all 50 states would be tricky to ramp up in a few months.
The key date to watch is May 9th. That's the deadline to get on the Texas ballot; a state that would seemingly be crucial to a Trump Independent campaign.
There is both *on the ballat and *write-in eligible. They are generally 2 separate issues.