Near the end of secondary school, the teachers used to print out tonnes of exercise and exam papers for us to practise. My school bag usually weighed 5kg (yes, I weighed it out of curiosity) just full of papers.
I noticed that most of the papers only had words printed on one side, so the other side was blank. I thought it was a waste to throw all these papers away, so I stored them all and decided to use the blank sides whenever I needed to scribble to-dos, shopping lists, study notes, diagrams, impromptu recipes etc. When I babysit kids and they want to draw, I just give them the used paper. I've also been keeping all papers I get through the post that are blank on the other side.
It's been three years since and I haven't been able to chip away at 5% of the papers I stored. Besides one pack of A4 for the printer, I won't need to buy plain paper for years to come. Honestly if you want to significantly reduce your environmental impact and save money then save all the existing papers you get that have a blank side.