Start with a qualitative interview survey to make inductive findings then use these to design questions for a follow up quantitative questionnaire survey
Start with a quantitative questionnaire survey, including asking them whether they would be willing to be interviewed, then follow up with interview questions on the results that are conflicting or unexpected.
This book is quite comprehensive when it comes to clarifying different avenues to mix quantitative and qualitative methods: https://www.oxfordhandbooks.com/view/10.1093/oxfordhb/9780199933624.001.0001/oxfordhb-9780199933624