Here is the process to create Coupon Code for any particular product:-
Login to Admin
Go to Promotions -> Shopping Cart Price Rules
Click Add New Rule
Fill Rule Information
Set Conditions
On left sidebar, click Conditions tab
Click + button icon
Select Product attribute combination
Click + button icon
Select SKU
Now, you will see SKU is ...
Put your product SKU over there
Please note that the SKU option may not show up by default. (Go to: catalog -> attributes -> manage attributes. Then search for the ‘SKU’ attribute and set the drop down ‘Use for Promo Rule Conditions’ to YES and reindex if needed.)
Set Actions
You can set how much discount you want to give from here
Save Rule