Follow this tutorial to make a DIY Sonoff smart switch:
https://www.instructables.com/id/Make-a-DIY-Sonoff-Smart-Switch-Use-Android-App/

SwitchIoT is a tools will guide you to make a DIY Sonoff smart switch based ESP8266/ESP8285 chip. Using this app for directly uploading firmware to ESP8266 then setup it no need computer/laptop for configuration.
Make an IoT device is as easy as uploading firmware without the need for registers/login and server configurations.

Features:
- Remote control
- Schedule
- Timer
- Loop timer
- Sharing
- Uploading firmware

Compatible Hardware:
- ESP-01
- Wemos
- NodeMCU
- Bluino Iot Kit
- Sonoff Basic R1/R2/R3/mini
- ESP8266/ESP8285 module