Create custom model for SITL

I followed the instructions. Here is my issue:

INFO  [px4] Calling startup script: /bin/sh etc/init.d-posix/rcS 0
Error: Unknown model my_vehicle (not found by name on /home/user/Desktop/File/PX4-Autopilot/build/px4_sitl_default/tmp/rootfs/etc/init.d-posix/airframes)
ERROR [px4] Startup script returned with return value: 256

I am new to these things but i checked the

/home/user/Desktop/File/PX4-Autopilot/ROMFS/px4fmu_common/init.d-posix/airframes

I can find the file which is instructions saying “4229_my_vehicle”

Can you help me about it ? Or are there any instructions for adding custom model(Couldn’t find properly) ?