How do you do a fishtail plait?

Like 'plaited,' yes.

for French people there is nothing 'French' in plaits. That hairstyle is not specially popular and is just called plaits tranlated in French as 'une tresse / des tresses'

a scalp plait is either a french plait or cornrows, this is where the plait is stuck to head.

get the hair that you want to plait, and tie the rest of your hair up so it doesnt get in your way you might wanna put some spray on it so it holdss then get a section from the top of your parting...