If work order scope is with material and services, it will be treated as Works Contract services and SAC will be 995416 and applicable GST rate will 18%.
As usual procedure, Contractor has to submit his PAN xerox, GST registration Certificate, ESI, PF registrations. In Work order has to made Contractor responsible for all the Taxes and Duties.
At the time each running bill has to submit his PF, ESI challans for last month. You have to deduct applicable TDS under Income Tax Act. 1% if he is Individual or HUF else 2%.
For GST confirm his filing of returns Form GSTR 1 and GSTR 3B prior to making payment.