Looking to build a variable form

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I'm looking to make a contact us form that the first question has a dropdown and the following questions change based on the selection of the first one.

For example the first dropdown could have the option of (general enquiry / shoe customisation ...) then if selecting 'shoe customisation' the following question is whats your size? with a checklist where 8 and 8.5 can be selected to help answer their question. 

Whats the best way to do this?