Living simply is, well, simple. It requires little more than cultivating a conscious effort to do no harm. Or at least as little harm as possible. All the rest follows from that effort.
It is hard to spend money these days and NOT do harm. Therefore, deciding to reduce the harm one does also means saving money. And any time we harm anything else we are actually harming ourselves in the end. But we can stop.
Think of all the things you would quit doing if you were trying to do no harm. You would eat less meat, fly less often (or not at all), and divest from all investments in fossil fuels, nuclear, weapons, and tobacco.
War and violence would be totally out of the question and would be relegated to the dust bin of history. These would be replaced with love, and we would become creators rather than destroyers.
Reducing the amount of harm one does leads to doing as much for yourself as possible rather than relying on corporations to provide for us. It would lead to things like cooking for yourself rather than eating fast and prepared foods. Self-reliance and building a supportive, loving community become priorities.
There are many, many other things that would help in becoming less harmful to the environment and all the things living in it. The possibilities are only limited by our imagination. The quest becomes even more exciting because there are always additional things one can do to reach a state of harmlessness, or at least as close as one can get.
Unbelievers like to point out things like, "Carrots feel pain when you pull them out of the ground", and they may be making a valid point - it is unlikely we can ever eliminate harm entirely (sorry carrots). But that does not mean we can't reduce it to an acceptable level.
Perhaps what simple living detractors are really saying is that consumerism is fun, and convenient and easy. Who would want to give all that up? And how exactly does one show their placement in a certain social class if we can't do harm?
The higher we go up the social ladder the more we tend to do harm. This is because we show our wealth by waste and destruction. The more we use and waste, the richer we appear.
If we can reduce the amount of harm we do in our short period on this troubled planet, why wouldn't we choose to do so?
It is simple.
Just cultivate a conscious effort to do no harm. The rest follows naturally.