How should I make an ECAD schematic for my RFID chip?
If it's on a PCB and all the components have pads, then usually a few files are generated with the component positions, a bill of materials (BOM) and reference designators indicate where all the components need to be. This is a good article on all the info to create a PCB. If you use Eagle cad or ki cad then the software is free.
The information on how to enter the information for the SL3S1203 can be found here, and the footprint looks like this:

However, if the above schematic is a modification for another board, or a board that has been already built the board house needs explicit\very exact instructions on how to make a custom modification.
Usually, I do the modification my self, provide a schematic and a picture and give step by step instructions on how to mount the components on the module. The instructions should be exact.