how do you buy and sell automatically

Based on the code you provided, you can use this modified code to create a Conditional Order that is triggered based on your code. There are two signals included here. One is triggered by RSI crossing above the threshold. The other is triggered by RSI crossing below the threshold. You will need to adjust the threshold and select which trigger to use, based on which direction you are trying to trade. Be sure that you thoroughly test using PaperMoney before applying this to live trades. 

input length = 14;
input threshold = 30;
input averageType = AverageType.WILDERS;
def rsi = RSI(length=length, averageType=averageType).RSI;
def crossingAbove = rsi[1] < threshold and rsi > threshold;
def crossingBelow = rsi[1] > threshold and rsi < threshold;
# use this plot to trigger on RSI crossing above threshold
plot trigger = crossingAbove;
# use this plot to trigger on RSI crossing below threshold
#plot trigger = crossingBelow;

New features have been enabled in Thinkorswim which provide fully automated round trip trades. Meaning we can now automate the entry and the exit and we can daisy-chain multiple stages within the order sequence so that multiple round trip trades can be accomplished automatically. In addition to this, we can save these orders as an order templates so they can be reused with very little effort.

Check the following video for the full details:

https://www.hahn-tech.com/thinkorswim-automated-round-trip-trades/