A rollover IRA is an IRA funded by assets from an employer-sponsored retirement savings plan. The reason it has to be designated "rollover IRA" is to notify the IRS that the high assets were accumulated in a plan with higher contribution limits and over the span of employment. There are no specific differences in withdrawing assets from a rollover IRA from any other IRA account.
Call the rollover IRA custodian or visit the office where your account was opened. Request "withdrawal" or "distribution" paperwork.
Fill out the paperwork making sure to note how much you want out and whether this is a one-time distribution or whether you are setting up a regular withdrawal program (i.e. a monthly income stream). Also note on the paperwork whether you want more than the automatic 20% federal withholding to be applied to the distribution.
Sign the paperwork and submit it. You may include your checking account information with some IRA custodians to have the distribution sent directly to your bank account.
Wait for the funds and confirm that the correct amount was distributed.