Take bus from Bukittingi to Payakumbuh and on to Harau Village, from there by motorcycle taxi to Harau Village. Call a homestay for a pickup and/or a tour around the valley.
It is small enough to walk around. Echo Homestay rents out bikes.
There are some smaller shops and restaurants at the entrance to the valley, but as you get deeper on the country roads there is nothing. Try to stock up on food before arriving. The selection is quite limited at the small shops. There is a nice restaurant by a pond about 1 km outside the park entrance.