How can I DCA a big sum of money now for investment horizon of 10-20 years?

1. TD Ameritrade is offering 0% commission now ( http://communications.tdameritrade.com/H/2/v400...). Perhaps you can consider using them so you do not have to worry about the fee.

2. If you go with IBKR still, then 10k a month for 14 months might be a better option since you do not want to overpay for the commission. But if you do switch to TD, then you can considering spreading your investment to even 5k a month for 28 months.

3. Robo-advisor is always a good place to start imo. My personal favourite would be StashAway (due to their low fee and personalized portolio). If you want to handpick some ETF, kristal.AI is good as well.

4. FSMOne offers good RSP with super low fee. Though the options available there are not superb, it does cover some of the bigger ones such as VOO, FTEC, CQQQ, etc.

The market will ALWAYS be volatile in the short run. You need to understand that the stock market goes up and it goes down periodically. It is part and parcel of stock market investing. What you should really do when you don't really know what you're doing is to just buy a ETF on a CONSISTENT basis by not caring about price and switching to a LONG TERM mindset.

If you are too concerned with price, you do not have emotional discipline and you're better off investing through a Robo Advisor or get a Unit Trust.

If you are cost conscious, DCA every quarter and put in a larger sum of capital every time you DCA. Set up a GIRO on a fixed date so emotions will not come into play. It is not the method or ETF that kills investments, but really the human brain and our very human nature which lead us to irrational decisions.

Hope this helps.