Knitting top-down, set-in sleeves is a technique I learned years ago, and from that first time, it became my go-to sleeve construction.  This is the best way to get a perfectly fitted sleeve around your shoulder and arm.   It occurred to me that many experienced knitters aren't familiar with this simple and fool-proof construction.  This sleeve technique can replace any standard sleeve that has a bell shape at the top.  I've only used it with stockinette and garter, but I think with a little bit of planning, this technique could be used with more complicated stitch patterns.  

The sleeve scythe, or the opening where the sleeve is set into the body,  is totally customizable.  If this is a sweater you will be wearing right next to your skin, you might like the look of a more fitted sleeve.  If this is a sweater you will most likely be wearing over a shirt,  such as a cardigan, you'll likely want the opening a little larger to accommodate for that. Top-down set-in sleeves give you the freedom to change these measurements on the body of your sweater, but your sleeve will need no adjusting; your sleeve will automatically fit!  You'll never have too much sleeve and not enough sweater or vice versa,  avoiding any potential puckers around the sleeve scythe once and for all.  And, this sleeve will fit your arm too, because you can try it on as you go along.  

You can either knit your sleeves in the round or flat.  If you knit them in the round, seam both sides.  If you are going to knit them flat, leave them un-seamed.  For both, seam the shoulder seams.

Before you proceed, please read the entire directions below.

Start with the right sleeve, and using a circular needle with the right side of the back facing you, start at the underarm and pick up stitches all around the back arm scythe to the shoulder seam.  The ratio is 1 stitch per 2 rows, even at the underarm.  Pick up the same amount of stitches down the front arm and underarm.  If you have 30 stitches up the back, you will need 30 stitches down the front.  Now the knitting begins.  Turn your work and with right side facing, begin row 1.

If you are familiar with wrap and turn short rows, you're probably used to "picking up the wraps".  In this sleeve technique, you DO NOT pick up wraps.  This gives a more "full-fashioned" look to the sleeve, which I prefer, however, you can pick up the wraps if you prefer that look.

See the pictures below for help with each step.  I hope you give it a try!  I bet you'll find yourself using it as often as you can!  (Please see this post for better pictures!)