Steps to create custom IDOC

Scenario:  Let's assume we require to create a custom IDOC for posting accrual (FBS1 transaction). For posting accrual we have a BAPI called 'BAPI_ACC_DOCUMENT_POST'. Our first approach will be to understand the transaction FBS1 and identify the mandatory and minimum fields require for posting accruals by transaction FBS1.
First we will be making the segments in custom Idoc  with same fields require to populate in BAPI. Our approach for posting  accruals from custom Idoc will be, first we will populate the segments of custom Idoc and then we will pass those fields to our 'Z' function module (which we  will be creating with BAPI functionality). Our 'Z' function module will be doing the whole functionality for posting accrual.
Steps we will perform in creating custom IDOC:
1>     WE31: Creation of segments (ZFBS1_HDR, ZFBS1_DET).
2>     WE30: Creation of Idoc type (ZFBS1_IDOC_TY).
3>     WE81: Create message type (ZMGFGS1).
4>     WE80: Assign message type (ZMGFGS1) to Idoc type (ZFBS1_IDOC_TY).
5>     SE80: Creation of function group (ZFG_IDOC).
6>     SE37: Creation of 'Z' function module (ZFM_ACCRUAL_POST).
7>     WE57: Assign Idoc (ZFBS1_IDOC_TY) to function module (ZFM_ACCRUAL_POST).
8>     BD51: Create an entry for function module (ZFM_ACCRUAL_POST).
9>     WE42: Create inbound process code (ZFBS1POST).
10>  BD54: Create logical system (CIDCLNT200).
11> SM59: Create RFC destination.
12> WE20: Create partner profile.
13> WE19: Test tool for Idoc.
Step 1: WE31: Creation of segments (ZFBS1_HDR, ZFBS1_DET).
Create header segment


Step 2 : WE30: Creation of Idoc type (ZFBS1_IDOC_TY).



Step 3: WE81: Createmessage type (ZMGFGS1).
Step 4: WE82: Assignmessage type (ZMGFGS1) to Idoc type (ZFBS1_IDOC_TY). 

Step 5: SE80: Creation of function group(ZFG_IDOC).
*
Step 6: SE37: Creationof 'Z' function module (ZFM_ACCRUAL_POST).*
Copy any standard function module which is running behind any IDOC, here I am copying 'IDOC_INPUT_FIDCC2' to 'ZFM_ACCRUAL_POST'. Comment the standard code and activate the function module.


Step7: WE57: AssignIdoc (ZFBS1_IDOC_TY) to function module(ZFM_ACCRUAL_POST).



Step 8: BD51:Create an entry for function module (ZFM_ACCRUAL_POST). 

Step 9: WE42: Create inbound process code (ZFBS1POST).
  
Step 10: BD54:Create logical system (CIDCLNT200).
    
Step 11:  SM59: Create RFC destination.
Step12: WE20:Create partner profile.  

 
Step 13: WE19:Test tool for Idoc.




Fonte: https://wiki.scn.sap.com/wiki/display/ABAP/Steps+to+create+custom+IDOC