difference between plan and procedure

A plan is how you are going to Perform the Management of the Schedule and the Schedule Procedure are the detailed steps to carry out the plan.

for example The Schedule Management Plan requires monthly updated schedule submission. This is a plan.

The Schedule Procedure describes in more detail what this entails such as progress shall be recorded thru the 21st of each month. Actual start dates, Actual finish dates should reflect actual progress. Activities that are in progress should have their actual start date recorded and the remaining durations should be adjusted to reflect the required durations to complete the activity. All updates must be submitted no later than the 3rd of each month.

The above is a procedure

Standard operating proceedures reduce the complexity needed in a plan, see: http://www.mosaicprojects.com.au/WhitePapers/WP1086_Standard_Operating_Procedures.pdf 

Thanks a lot Zoltan and Patrick