Regarding idoc to EDI conversion process,mapping,basic services to use,search in this site you will see the results.
Also go thru the SAP Adapter userguide,EDIModule userguide for more information,it helps to move further.Also look into the WmEDISamples package in the Developer tool about mapping to X12 structure and utilities etc…
Which version of webMethods are you using?
If using IS6.x then once you installed EDI module basically you need to install the EDI X12 4010 doc types in TN and this will automatically install the EDIFFSchema in th WmEDIForTN package.
If using IS4.6 then download the template(4010) from Advantage site and follow the procedure for installing,it is documented in the EDI userguides.