It sounds like you are asking how to place an order, not what ETF to use as well?
I use TDA.
Order type, Market will be the current price the ETF is trading at. Good on most normal volume trading days but not good if the market is volatile. I always use the Limit order, you enter the price you are willing to pay and you will get it at that price or lower, UNLESS the ETF is trending up, no one will sell you the ETF below the current market price.
Time in force. If you want to buy at the current market price then Day is fine. If you are trying to get a better price (lower) and are willing to wait some time for the market price to come to you the then use GTC, Good unTil Cancelled.
When I am ready to buy something I'm not looking to nickle and dime the price, I'm in it for the long haul. I enter a price slightly higher than the Last Trade price and select Limit order and Day, then Review order, make sure I didn't goof something and hit submit.