I am getting struck on how best to do fractional stitches

by bernadine wray
(manchester, England)

I am doing Pluto disney character on a cushion. I have got to the stage where I need to add fractional stitches. I have got a lot of information on how to do it. I am not sure if I should do one fractional stitch at a time and then cast off. Should I do a loop to start the stitch. What number crewel needle should I use. I have a packet of crewel needles and they say on the outside size 5/10, size 3/9. How do I know which size is which.