These are the steps for custom idoc for outbound as well as inbound.

• WE30 - IDOC type creation 

• WE31 - Create segment

• WE81 - Message type creation

• WE82 - Link IDOC type to Message type 

• SE37 - Create Inbound Function Module 

• BD51 - Maintain entry for Function Module

• WE57 - Maintain

• BD57 - Link Function Module, IDOC type and Message type 

• WE42 – Create Inbound Process Code

• BD67 - Link Process code to Function Module 

• WE20 - Create Partner Profile

• BD64 - Display Distribution Model

• WE02 - IDoc List, Display all Inbound/Outbound IDocs

• WE14 - Outbound Processing of IDoc’s

• BD20 - Inbound Processing of IDoc’s
 

1st Step: Create a Segment (WE31)
Segment is a structure for passing data in IDoc. It actually contains the IDoc data, just like the DDIC table/structure. Segment is created with all the required fields. Then Save it. But, to actually use this Segment, you have to Release the Segment, otherwise u can’t use the Segment, by menu, EDIT--> Set Release.

Now, if you want to do some change to this Segment, you cant, unless & until you Cancel the Release, by Edit--> Cancel Release.
 

2nd Step: Create IDOC Type (WE30)
After creating the Segment, now we have to create the IDoc Type. IDOC Type is like an envelop of a letter, which contains the data inside it, & also some more information like address. IDoc type can be Basic Or Extended.

Basic IDoc Type: 
Using some SAP existing IDOC type (ex. MATMAS) or Custom IDOC type.

Extension IDOC Type: 
When we need some fields extra to an existing IDOC type, then we can extend that Basic Idoc Type by another Segment. This is called Extended idoc type.
 

3rd Step: Create Message Type (WE81)
Message Type is like the Postman for sending the Letter.
 

4th Step: Attach Message Type to the IDOC Type (WE82)
 

5th Step: Create a Function Module (SE37) 
Write the Processing logic in a Function Module.
 

6th Step: Mention the IDOC Type, i.e, 0/1/2 (BD51)
 

7th Step: Assign the Message Type, IDOC Type & Function Module (WE57)
 

8th Step: Create a Custom Process Code (WE42) 
 

9th Step: Attach the Function Module to the Process Code (BD67)
 

10th Step: Execute Inbound IDOC’ s (WE19) 
Example Function Module: IDOC_INPUT_ORDERS (Standard FM to create Sales Order).

Get help for your Basis problems
Do you have a SAP Basis Question?

SAP Basis Admin Books
SAP System Administration, Security, Authorization, ALE, Performance Tuning Reference Books

SAP Basis Tips
SAP BC Tips and Basis Components Discussion Forum

Administration In SAP - Sapgui, Unix, SAP ITS, Router, Client Copy and IDES 

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com