Before you start playing sweep picking consider this:
1) Your palms position (For comfort, mute strings ...)
2) Your plectrum (make sure it is comfortable for you to do sweep picking)

And like other people say, practise makes perfect and it is true. Start from slow tempo and increase tempo speed slowly and if you got off tunes or breaks off... start from the begining again. Slowwww to Fasssttt....