1. Go to the Dollar Tree. This is very necessary, for obvious reasons.
2. Buy some of the less plastic looking bunches of leaves, leaf garlands, and some spicy candles for good measure.
3. Go home and paw through the stuff you already own. Pull out things like old books, those wooden candle holders you know you have stashed somewhere, and other things that look somewhat autumnish. Maybe collect a few sticks or berries outside as well, if you live in the country. 
4. Throw the leafy things and candles you just bought, your old books and candle holders, and the sticks around the room, aiming for the emptier spots on your shelves. However they land is great, since we all know asymmetry is very necessary for pleasing visuals. Smile and tell yourself what a great job you’ve done. Consider starting a home decor blog.
5. You know those friends you have whose homes always look like Pottery Barn and Pintrest and IKEA catalogues? Don’t invite any of them over for the remainder of the autumn season. Note: This step is very important for peace of mind. Also, for maintaining your opinion of your awesome decorating skills, don’t visit your classy friends at their homes either.
6. Bonus step (Not pictured): Dimming the lights does wonders for making all those plastic things look less fake. This is where those candles you bought come in handy. Now grab one of your books, sit on your blue couch with a fluffy blanket and a cuppa tea, and enjoy the view.

Confession: Some of the things purchased may have been bought at more expensive places than the Dollar Tree, such as (gasp) thrift stores or yard sales.