BY far the best time of day to do this is upon arising. If you are into the relaxation response, you are getting 24 times as much reversal of the fight or flight storage in the hypothalamus as if you were asleep. Therefore getting up a half hour early will have you a lot more rested at the time you would have regularly gotten up than if you had stayed asleep. That takes care of one of the two times you need per day. If you cannot find one 20 minute period a day for this, you need to look hard at your priorities.
Many people find that tapes work.
The workbook gives other options for those for whom that is NOT the best way. I have just been notified that the earlier editions are MUCH more helpful than the later ones. Ask your librarian for one from the late '70s. You can do it for an interview, while you work, while you drive, etc. You will be much more effective in anything you do while in that rhythm.