Main guide : https://openwrt.org/docs/guide-user/additional-software/imagebuilder
Colab Link for easy building : https://colab.research.google.com/drive/1jZ9FmreMcMrBeG9Xo9QWLRKzVLg1B4bb?usp=sharing
I used this since I needed mwan3 and opkg install wasn't working because of the limited flash memory of the router.