This is the another option (easier one) for creating it using CRM developer toolkit which comes with the Dynamics CRM 2011 SDK.
General knowledge about CRM developr toolkit
Other good step by step blog for building plugin using developer toolkit
Creating plugin using developer toolkit:
1) Install CRM developer toolkit
2) Create new developer Project
3) Create new plugin project. The great thing about it, it comes with all the relevant references in it.
4) Generate the wrapper (CrmSvcUtil)
5) Choose the entity you wish to add plugin for
6) Choose plugin configuration
And that's it, Start writing plugins..