Need SQL help

Could you please help me with the below steps all in one table?

Step1 : derive the last day based on SHIPMENT_DATE .
Step2 : derive the date_add_sp = DATE_ADD('DAY',DAYS_SUPPLIED,SHIPMENT_DATE)
Step3 : find the last day of the month based on date_add_sp
Step4 : date_diff_dp = calculate the difference of date from step 3 and date from step 1 in months

Thanks in advance.