How to buy and sell put options if i think the stock is going down

Hey Guys, i'm getting into option trading and i wanted to get this clarified. If i was expecting a stock to go down and i wanted to get into a PUT position. After selecting a position on the option chain, do i “sell to close”, “sell to open covered”, “buy to open”, or “buy to close”? and to exit the position if the stock goes down for a profit, which one of those do i choose, and do i pick a position on the option chain again where i want to sell?

And when i'm in the position, is the stop loss margin auto placed for what i invested only? so i can't lose more than what I invested?

I'm using TD fyi, and my god all the videos online about this are awful. Everyone's giving opposite answers and using different phrases to explain all this.