You don’t need candles, but millions of people buy them. So if you do, choose ones made from biodegradable plant wax, which is also safer and cleaner/slower to burn. Use with recycled candlesticks.
Follow candle safety tips. Keep away from pets and children (essential oils including citronella can harm), air rooms after use and never use in bedrooms nor leave unattended. Keep away from flammables and put out with a candle snuffer.
Trim wicks to 1/4 inch before lighting and keep free from trimmings and matches, and don’t burn more than a few hours at a time, stop use with 1/4 inch of wax remains. The first burn, ensure it’s long enough for the wax melt pool to reach the edges, this will prevent tunnelling. Use on a stable heat-resistant surface, and never burn to the bottom. Keep reed diffusers out of the reach of pets and children, or places where they could be knocked over.
Try to use soy wax (burns cleaner and slower with fewer soot particles) or cleaner coconut wax (rapeseed wax is also locally grown). Choose unscented or real essential oils. And try to find brands with organic cotton wicks, in sustainable packaging.