Hi Dimple.

A Site Instuction given by Client should be shown in the schedule by putting the duration of the same. The duration can be determined by additoning up the Duration for E+P+C= Your Engineering Manager can give you the duration for Completing the Engineering + Procurement Head can give you the Duration for Delivery of Item + You can calculate the duration for Construction using the productivity Norms & dicussing the same with the PM & CM. So that would be the total duration of Site Instruction (You can delete any of the above E or P or C if not applicable).

Note : It would be better to add some buffer in the duration like 2 or 3 days.

So you have to include new activities in the schedule related to the Site Instruction and link it to all the baseline activity which is impacted by this Site Instruction. 

If it is impacting the finish date of the project issue a Variation Notice, so that the Client knows that the Site Instruction is impacting the Finish Date of the project. 

So the Client will either accept the impact or will instuct a mitigation plan or cancel the Site Instruction.

Regards

Jithin