Can someone help me this part? It just hard for me to understand and not sure how to do it.
The total sts I have on my needle is 86 sts.
When work meas 49cm from beg, cast off 12 center sts, then cont on 1 side at a time. On neck edge, cast off 5 sts once, 4 sts once, 3 sts once, 2 sts once and 1 st 3 times.
Mark off the center 12 sts, work across the RS row (right side) to those 12 and bind them off. Continue to work to the end of row, turn and work back to the gap before the bound off sts.
Turn to the RS and bind off 5sts then finish the row and the next WS row.
Turn to the RS and bind off 4sts.
Repeat these directions on this shoulder.
To work the other shoulder, attach a new strand of yarn, bind off 5sts at the neck edge on the WS row and proceed in the same manner. This time the bind off will be at the beginning of the WS rows. All the bind offs are at the neck edge.
ohh. My next working row is WS. Will that be still good? Or I need to undo all my previous rows and make my next row to RW?
You can work one more row so that the next row is a RS row.
Oh…I still need to work the piece till the measurement meet 49cm from beginning before start the mark off the center 12 sts part…, right?
may I ask why it is important to start on RS row? Is there something I need to take note for future project? 
Thanks
You could work this from the WS row unless there is something else in the directions that follow that requires working from the RS row at this point. It may be more of a convention of pattern writing that neck bind offs start from a RS row.
Yes, work to 49cm first, then start shaping the neck.
TY Salmonmac.