Shower. Whilst in their shave and exfoliate really well. Make sure to wash off any deodorant, the tan will turn green if it mixes with deodorant.
Usually I do my prep the night before tanning otherwise the tan can stick in your pores and leave little black dots where you've shaved.
If you want to do it straight away then finish off with cold water to seal pores.
Don't use any oil based shower gels. Bar soap is best.
Get out, dry off fully and lightly moisturise ankles, knees, elbows and anywhere you've particularly dry skin. Wait for it to sink in properly.
Use the mitt and start, with sweeping motions, from the bottom up (legs, torso, back, chest, arms..)
Lastly, I use a makeup brush to apply tan to my feet, hands and face. It adds a lighter coat. If you use the mitt be very careful, it could add too much and your hands and feet tend to really cling to tan. Makeup brush is definitely a better idea.
Wait until your tan is dry, wear loose clothes to bed and wash it off in the morning.
To build colour you're better off doing several light coats of tan, instead of one 'heavy' coating. It can end up smudged if you use too much.