Washing a cotton garment

I knit a vest out of cotton thread. Is it better to wash it before assembling or assemble it first? Also, what is the best way to get it to shrink in the width only? Any suggestions?

Cotton doesn’t always shrink, I’ve found they stretch with wearing and go back to the knit size with washing as long as you don’t leave them in the dryer too long. Things that shrink sometimes get wider, not narrower. If it’s too large for you now, it may not end up the size you need unless you redo it.

What would be the best way to make it the right size without having to redo it? Can I just overlap the sides when I assemble the pieces?

It depends on how thick the sweater is. If you make the seams too big, you may just end up with extra bulk at the sides which you may not want. Here’s one method of cutting (scary but do-able) that was used to decrease the width of a sweater. The early part of this link is about making sleeves and body shorter but if you persist, the author gets to steeking or cutting the sides. You might test this out first and see how it goes before you try it on your sweater.