Just including instructions in a Will does not ensure that your wishes are carried out. Often Wills are not actually read until after the funeral has occurred.
The challenge before you is to make sure that your wishes are clear to your family, friends and anyone else that may be entrusted with taking care of your funeral arrangements.
Written instructions should be left with your personal papers. You might also consider pre-arranging or prepaying for your funeral. This helps to clarify what your wishes were and diminishes the need for family members to make a decision about whether to cremate or bury your remains.