The Standard Operation Procedure (SOP) for purchasing depends on the rules and procedures set by each company. Based on my several years of experience in this field, I would like to share with you the following steps that I usually practiced.
1) Get the complete description of the materials that you are going to purchase.
2) Canvass them from at least 6 credible suppliers.
3) Ask samples of the materials for quality assurance and low cost of course.
4) Negotiate for the lowest price without sacrificing the quality.
5) Make sure that the delivery of your orders will be made promptly.
6) When receiving the deliveries, make sure that the description, quality and quantity of all items indicated in your order slip are properly complied,