Tried fixing turning of elastic at the back

I tried to fix the back of this skirt based on the perfect (or so I call it) phus phus skirt I was creating.
The back of the phus phus skirt is both gathered and elasticised to give that flat look but when I created this skirt I had fully elasticised the back.

Now when I tried doing this following step 17 of phus phus skirt I just couldn't get the fabric over the waistband fully straight even though I used all my energy with both hands.
Also when I'm behind the sewing machine I cannot pull it so taunt like shown here.
Had I gathered it slightly I would be able to stitch on top of elastic laying the fabric flat.
Do I didn't go ahead with my plan but just wanted to show that a gathered and elastic is the way to go :)

Thanks,
Preethi.

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