Depends on exactly what you're trying to do.
Is it something like a hat that the pattern is written knitting it flat and seaming it and you want to knit it flat on circlular needles? Easy - cast on your stitches and turn at the end of each row, just like you'd do it on straight needles.
Or is it something like a hat that the pattern is written knitting it flat and seaming it and you want to join it and knit it in the round? Depending on what the item is, you might need to drop 1 or 2 stitches off the cast on that would have been used for seaming. Just cast on your stitches and join in the round.
Mirl56 at Ravelry, too.
Learn from the mistakes of others. You canít live long enough to make them all yourself. Eleanor Roosevelt
Lady Feb Sweater
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