There is a considerable number of MPs who support a confirmatory referendum on the deal. What legal options do these MPs have now? If there are several possible strategies they can pursue, which one is likely to be the most effective?
At the end of the day, a bill setting out the terms of a referendum must be passed by Parliament before a referendum can happen. With the original 2016 referendum, taking into account time for the Electoral Commission to test various forms of the question, that took several months.
If they wanted to go down this route, they can use similar processes that they have in the past (generally under Standing Order 24, which covers emergency debates) to introduce a bill that would require such a referendum.
It is also worth noting that we are, as of this writing, 11 days away from the UK's departure from the EU. While the UK Prime Minister has submitted a letter requesting an extension, there is no guarantee that the EU will agree to one. If they indeed decline to do so, then there is no way for a referendum to be organised before Britain's departure (apart from withdrawing the original withdrawal notice, i.e. "revoking Article 50").