Peripheral GATT design

[saeed mahvis]

Hi

I want to design a GATT service as a connectable peripheral device. My  board is NRF5340-DK. 

The software which I am using is NCS(based on Zephyr).Which sample code I should use as

the starting point?

[Kenneth]

If you want to get started with nRF Connect SDK you can check out:
https://devzone.nordicsemi.com/nordic/nrf-connect-sdk-guides/b/getting-started/posts/nrf-connect-sdk-tutorial 

If you are familiar with nRF5 SDK you may want to check out:
https://devzone.nordicsemi.com/nordic/nrf-connect-sdk-guides/b/software/posts/building-a-ble-application-on-ncs-comparing-and-contrasting-to-softdevice-based-ble-applications 

An overview of existing sample projects:
https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/samples_bl.html

As a starting point I would likely suggest:
https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/bluetooth/peripheral_lbs/README.html 
OR
https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/samples/bluetooth/peripheral_uart/README.html