Make sure it's been 10 days since the town was built, and spend at least 8,000 Bells in Able Sisters. The next day, construction will take place next to Able Sisters. This shop will be Kicks, the shoe shop. Ten days or more later, spend at least 10,000 Bells at Able Sisters or Kicks. The next day, Able Sisters will be under construction, and Shampoodle will be open the next day. Getting a Mii Mask will cost 3,000 Bells. Make sure you select "Makeover" when you're asked what you want.
You need to unlock the shop on main street titled "Shampoodles". Sit in the chair that allows you to buy things, the one nearest the side the camera is at, and it should be pretty straight-forward from there.
