shifting from regular to composition

sir. let us say under regular scheme i have stock of rs.100 where in input gst is of rs 18. now i sold rs. 50 stock for rs.100 and the input of rs.18 was set off against output gst of rs 18. now i have another closing stock of rs 50 and accordingly i shifted from regular to composition. now what are the formalities to be completed? kindly advice.