I am not sure if I am answering the question about your sashing correctly or not but since you are a newbie-if not hopefully you can use this info at another time.
Before you sew on your sashing you need to measure carefully your block.Your sashing pieces should be the same size as your block.EXACTLY!!! I only put sashing on my sides and use long sashing pieces for tops and bottoms once I sew all my blocks together.
But the most important thing is to measure,measure,measure.
Would have put smiley faces on but I don't know how.