Switching from straight to circular

How do you switch to circular needles when pattern calls for straight. The pattern gives you directions to cast on for front work that, then cast on for the back and work that. How would I use circular needles instead?

Link to your pattern, please? Are you planning to work front and back flat at the same time? I’d use one ball of yarn and cast on for one piece and another ball or the other end of the same ball to cast on for the other. You then work back and forth - row 1 on one piece, drop that yarn, pick up the other yarn and work row 1 on the second piece. When you turn your work you’ll do row 2 on the same piece, drop the yarn at the end of the row and work row 2 on the other piece with its yarn. So on, and so on.

Just be sure to change yarn when you start knitting the other piece. Putting your work down between pieces can make it very hard to know which way you were going so try to knit at least a few stitches on the second one before you answer the phone or whatever.

HTH

Sorry, my pattern is from a book. Maybe, thats why I’m confused. Could I just work the front with the circular needles same as if they were straight? and then do the back the same way. Does it really matter that they are circular and not straight when this way?

If you’re doing one piece at a time you could use straight needles if they’re long enough or circulars. Your choice. Your project, you can do whatever works for you.