I agree with OSS, best to start with what the pattern says. You can try running a line of long machine stitches round the sleeve head and pulling them up a little to form a very slight gather and that will help ease the sleeve into position. If there's a bit of give in the fabric, say cotton, linen or viscose you should be able to get the sleeve in ok. Also helps to take in some of the fullness if you can iron the sleeve head over a tailor's ham once you have sewn it into position.