and I thought I knew what I was doing....

I am trying to sew a smock style dress and am running into a few problems. In the past I have sewn some costume dresses and jumpsuits but because they were for the kids to play in, I left out the interfacing, didn't prewash anything, and wasn't so concerned with making sure I built a garment made to last multiple wears and washings. But now I would like to sew some new items for spring for myself and DO want them to last through multiple wears and washings. So i have a few questions in general and then specifically related to the pattern I am working with.
1. I need to prewash my fabric, is the best way in the washing machine or just in a sink of hot water and some detergent?
2. how do i wash my interfacing? will washing harm the fusibility of it?
3. can someone please PLEASE explain what a "double stitched seam" is?  the pattern instructions claim to define all bold terms but did not define "double stitched seam". Does the second stitch go to the inside or outside of the first stitch? how far from the first stitch does the second stitch go?

thank you!!

pre-washing should be done the same way you'll wash it after the item is made...if you'll do it by hand, then prewash by hand, etc. As far as the interfacing - washing won't hurt the fusibility, however, don't put it in the dryer...that would result in an interesting load when the drier is finished....

That being said, if your interfacing is 100% poly, as many are, you don't need to prewash at all.  As a matter of fact, if your fabric is all poly, it won't need to be washed, either, unless you want to get the sizing off of it before sewing.

I won't venture a guess on the "double stitched" seams unless I could see it in the context of the instructions. You probably can go to the pattern company website and send in a question to them.

Try "googling" your double stitched seam.  Here is just one link that I pulled up:

http://www.burdastyle.com/techniques/double-stitched-seam