Shaping shoulders of sweater, stuck?

I’m a beginner knitter (just started this summer) and I’m making my first sweater now. But I got to this part of the pattern and am a bit stuck, and unsure of how to proceed. I’m supposed to be knitting the shoulders, to have space between the neck on the front and back, and have just done this line:

[B]Neck Shaping: On the next right side row, knit 19 stitches, bind off the center 24 stitches, and knit last 19 stitches to end.[/B]

However, now I have the two sides of the neckline, where the shoulders will be, and the pattern says:

[B]Right Shoulder: Purl 15, knit 4. Knit across next row. Repeat last 2 rows until piece measures approximately 23 inches from beginning ending with a wrong side row. Bind off knitwise and loosely.

Left Shoulder: With the wrong side facing, join yarn at neck edge. Knit 4, purl to end. Knit across next row. Repeat last 2 rows to same length as Right Shoulder, ending with a wrong side row. Bind off knitwise and loosely.[/B]

I’ve done the p15, k4 for the right, but my other needle is holding the left side and I’m not sure if I’m supposed to use a holder to keep them separate and just work on the right, then switch over to the left, or if I’m supposed to do something else. If someone could clarify for me, so I don’t completely mess this up, I’d be really grateful!! Thanks so much!

You can put the left shoulder sts on a holder if you like while you knit the right shoulder. You could leave them on the needle and just ignore them while you finish the right shoulder. Sometimes it’s helpful to join a second ball of yarn to the left shoulder and work it at the same time as the right, using separate balls of yarn for each.
It’s your choice here.