Step # 1 Acknowledge that God is the real creator of your job offer.
Nothing happens without God's will and every single act of creation is being carried out by Him. Your job offer was the result of God's mercy. The first step to thanking God is acknowledging Him, His power, intelligence and His mercy. Thank You Lord for the job offer as nothing happens without your will. Thank You for making an impossible task possible.
Step # 2 Acknowledge that you were not entitled to get the job offer. It was only the result of God's mercy and favor.
First off, ask God for forgiveness for our entitlement mentality as it fools us into thinking that we deserve all the success, all the happiness, health and we don't even know why. It is our ego. So tell God in your gratitude prayer that you didn't deserve the job offer - it was only the result of God's mercy on you.
Make a mental list of benefits this job offer is going to create for you and tell God how thankful you are to Him for these benefits and blessings coming your way. For example, steady income on a monthly basis, end to unemployment, you can buy your family food, clothes, healthcare and other basic things for survival, more opportunities for employment as this job offer is sure to give you experience and skills. Charity (you can spend money for the needy, helping others). One by one, dwell on each benefit and tell God how grateful you are to Him for this favor (job offer).
Step # 4 Thank God for people and things that played their role in job offer
Thank God for your potential employer who offered the job, people who brought you this offer, newspaper, internet and advertisement through which you came to know about this job, vehicle that you used to travel to the company. Thank God for your education, your parents and family who taught you right from the start, your teachers who enabled you to read, write and speak. Think about other things/people that God used as tools to bring you this job offer. You can list an unlimited number of these things and people.