Help Can someone tell me how to work a piece on to the same needle?

I am a newbie and my pattern says to work sts from left shoulder on to the same needle. I have completed the front left, front right, and left shoulder pieces and have the right shoulder on the needle, but am not sure how to “work the left shoulder piece onto my circular needle” to continue the vest. Also this is worked from top down. Can anyone help me?

What pattern is this?

Sounds like you are going to be joining all of the pieces so you can continue top-down (? in the round).

Does the next part of the directions say to work across ALL the sts, or give a # that would total your right/left together?

You likely just need to move the L sts from whereever they are (? st. holder) to your needle so they line up in the correct order with your R sts. You may need to move/slip sts to get to the correct spot on your circular needle to do this.

Hi and welcome!
You may have to work different sections with separate balls of yarn. Can you give us a link to the pattern or a pattern name?