Informed consent
Agreement to take part given before anything is collected, by somebody who has been told what taking part involves.
The substance is what a participant needs in order to decide: the purpose, what they will be asked to do, roughly how long it takes, any compensation, any risk, what happens to the data, who to contact, and that they may decline. Ethics review bodies differ on wording and on what needs written approval; the underlying requirement is the same.
Place the consent screen before the questions it covers. In Plumeform, declining the consent gate records no response or partial response.
Withdrawal has limits worth stating honestly: once responses are anonymous and pooled, an individual's data usually cannot be found and removed. Say so rather than promising something the design cannot deliver.
Where it goes wrong
A consent statement written as the first question of the survey. Answering it is already participation, which is the thing consent was supposed to precede.