In the same way that an artist starts with a fresh canvas before painting, you want to start with clean hair before cutting.  Hair that's dirty or full of product will not allow you to see a true finished product when your haircut is completed.  Use a shampoo and conditioner made for the specific hair type.  There are products to fit every need--color-treated hair, dry hair, damaged hair, thinning hair, hair that lacks volume oily hair, normal to dry hair, and natural hair—so pick the one that's best based on hair type and needs. If hair is prone to knots or tangles (such as chemically treated or bleached hair), use a de-tangling conditioner or spray on a de-tangling product.  You don't want your comb to get stuck when you run it through hair that has mats or tangles. Make sure hair is dry if you plan to use clippers or if you want to carefully thin the hair to make sure you don't remove too much fullness.  Do a dry cut if you're simply trying to remove split ends; they're easier to see when the hair is dry. If you're pressed for time, a dry cut saves the need for washing and drying.

TL;DR in Indonesian:
Cucilah rambut. Potonglah rambut saat kering dalam kondisi khusus.