Fit the dress, and then while it's on the person, measure the distance between the closure that doesn't have the room to quite close. Make a note of the amount it needs to be let out and how far down the alteration needs to go.
Look at the least invasive place to let the dress out. If you have darts those are easier to adjust than say, letting out the side seams. You want to avoid letting it out around the zipper if at all possible.
Usually you'll want to adjust it in the side seams. Do this by opening any sort of lining or facing and then drawing a new line with chalk where you want the new seam to be (before you remove the old seam you will make the new seam!)
Make sure you take into account that when you are letting out a seam you are doubling the amount that you are measuring. If you mark your new seams 1/2" from the old seam line then when you open up the old seam you will have an additional 1" on each side seam and that makes for 2" total.
When you are confident of your measuring, sew along your new seam line and then remove the old stitches.
Press the seam open and do the same alteration amount to any lining or facing that will be affected.
Replace the lining or facings as you found them. Fit again. The dress should close nicely.
Steffani Lincecum is the author of 'Patternmaking for a Perfect: Using the Rub-off Technique to Re-create and Redesign Your Favorite Fashions', published by Potter Craft. Image from Mikhail Malyugin / Dreamstime.