Not bringing mains voltage into your project is the best advice. The only time you want mains inside a project is if you’re doing something directly with that voltage, like a light dimmer but those kinds of projects are simply not safe for those that do not understand the inherent dangers. Mains voltage will kill you. Period.
Use an external power supply, there is reason why wall warts and power supply bricks are available in every practical voltage with wattage. Seriously, buy a quality supply from DigiKey or Element14 or other reputable distributor in your country, don’t take chances with the Wung-hung-low crap from China. There are plenty of videos on YouTube, especially EEVBlog that will show you how bad and dangerous some of those devices can be.
If you must work with mains, you must properly ground (earth) all metal and ensure you switch the line, not the neutral. Maintain proper color coding (example: blue/brown/yellow-green) and always fuse the the device. Always heat shrink all connection points or use insulated crimp connections. Keep HV away from LV as far as possible, sometimes double insulation is required (tubing or heat shrink over twisted together switch wires as an example).
One last thought: Never use Chinese equipment as an example, something is wrong with that stuff about 99% of the time.