take your ukulele and strum down then strum up with your index finger but do it very very very slowly and concentrating on getting the up stroke sounding nice,dont worry about tempo just the action of the finger going up. now keep doing that for five, ten minutes, however long you can before it messes up. When yo're bored with that or the fingers are messing up the strum, put the uke down and go do something else (iron the clothes, clean the bathroom) then come back after 30 mins or an hour and start again with the strumming practice and try not to rush it.... just go real slow listening to the sound as it goes down and up. Keep this up for a day (BORING I know but it'll be worth it) and do it last thing before you go to bed. Next morning give it a go and I think you will find it has become better, not great but better?? Keep this up and increase the speed as you become more comfortable, but dont get crazy fast yet, keep it slow. And remember as soon as you're mussing up the strums, stop and go away...Practice and rest are both important