The roth conversion ladder is a thing people do to get money out of their retirement accounts before full retirement age (steps 1-3 below). I guess I was mostly just talking about a roth conversion (steps 1 & 2 below), but you could certainly do either.
Step #1: Call Fidelity and tell then you'd like to open a traditional IRA account with a Rollover from a 401k account. They can probably handle the particulars of contacting your original 401k provider and arranging the transfer. If not you may have to call your 401k provider and arrange it from that end.
Step #2A. Figure out how much income you could have in the USA this year without owing any federal income tax. (This should be at least $12,000, but might be more depending on the particulars of your situation.). Call Fidelity again and say you'd like to roll over whatever that amount of money is from your traditional IRA to your Roth IRA account.
Step #2B (not relevant in your case): pay any income tax owed on the amount of money transferred.
Step #2C: repeat steps 2A and 2B annually until the balance of your traditional IRA account is exhausted.
Step #3: Only if you want or need the money, starting five years after your first batch of money transferred from your traditional IRA to your Roth IRA, you can withdraw up to the same amount of money you transferred into the Roth IRA, completely (US) tax free, and spend it on anything you want (groceries, disco balls, pork belly futures). After six years, you can withdraw your second batch of money and so on.