Skirt as Big as a House

I have sewn up my test muslin for my skirt which I made from my personal body blueprint.  But when I put it on it was as big as a house...a good ten inches too big!!!  What did I do wrong???

In a case like this a little sleuthing work is needed.  So...now measure the paper pattern. Measure from CF to side seam - MINUS the dart space - then measure CB to side seam MINUS the dart space. Add the 2 measurements together and double it for your entire circumference. This should measure approximately 50" - (with, of course, the dart space eliminated). If it measures close to 50" - then the waist should fit.

Where are 10" extra coming from? If the pattern measures correctly then the fabric can't possibly be that big.  You are welcome to call me - I'm happy to help - maybe verbally we can figure out where the extra is coming from.
Kindly,  Glenda

I have laid out my bodice (which fits really really well and has all darts sewn in place) and my skirt pattern as a one piece dress as shown on the instructions and it really lines up perfectly.  So my mistake is some really dumb beginner mistake in the sewing.  All my seams are really good 5/8".

My waist measurement on the Front Bodice matches the waist measurement on the Front Skirt and as I said the vellum patterns line up almost perfectly.

My muslin test skirt is huge.  I held it up and measured it with a yard stick and one side measured 29" which is a total of 60"  and this was with all seams sewed at 5/8"..........HOLD THE PHONE......SCREAM!!!  I didn't sew in the back darts..........Grrrrrrrrrrr.  And I just checked my Back Skirt Pattern and it was drawn 2 sizes bigger than the Front Skirt Pattern.  OK this accounts for why it doesn't FIT!  I am going to go sit in the corner with my skirt over my head.