You have multiple options of how to create an LSMW project to maintain Article Master data within SAP.
You can use the IDOC method using message type ARTMAS Basic Type ARTMAS05, you can use standard input or direct input with object 020 Article Master Method 0000 - program RMDATIND and even BAPI with BUS1001001 Retail article
What you will need to do is work through each of the process steps within after defining the object attributes which include defining the source structure and fields (i.e. you flag file structure) and mapping the source structure fields to the target structure (I.E. IDoc fields) and then specifying the location of the flat file.
If you need more specific details you might want to check out SAP - Press.com website and search for the book Migrating Your SAP Data.
http://www.sap-press.com/products/Migrating-Your-SAP-Data-%282nd-Edition%29.html
there are websites that have some high level details but each project is unique you might not be able to get the exact layout of what you need but they can provide you with some starting places.
http://****************/Tutorials/LSMW/IDocMethod/IDocMethod1.htm