First create a source member that has the requirements for the LF. Fields used, select requirements, key fields..
Here is a sample of source for a LF called F$MACNEILC. This is stored in a QDDSSRC member with the same name as the file. The file uses all the fields from the PF, no selection of specific records, and is keyed by PARTNO
* RECORD FORMAT NAME
A R F$MACNEILR PFILE(F$MACNEILC)
* KEY FIELDS
A K PARTNO
Now at a command line key the command to create a logical file. CRTLF
Enter your new file name, and the library where you want to store the file. Usually the same place as the PF.
Enter the Source file and library (You just created this). If the member name is different, enter it, else you can leave it at *FILE
At the bottom of the screen, enter the name and library of the PF.
Put a + sign on the next line for Members. Press enter.
You now have a page where you can enter all the members names you need.
Press enter and you will create the file.