For two years, the project GRECO has conducted a survey to analyse the opinions, doubts and experiences of researchers on the field of engineering and photovoltaic (PV) energy about Open Science (OS). We gathered 106 responses, analysed them and came up with 39 Frequently Asked Questions on Open Science. The experience of researchers from the PV field is useful for other fields too. So, this document is a Practical Guide for Researchers and tries to solve the most prevalent doubts, concerns and fears. It starts with a checklist for OS, that researchers may consider in various phases of their investigation. Then, the reader will find a Q&A section, structured in nine categories: 1) where to start, 2) open data, 3) open access, 4) open to society, 5) other open practices, 6) property, 7) resources, 8) politics and 9) metrics. There are 3 to 5 questions for each category, and the responses come from experts on the field and other researchers that have tried and applied OS in its different forms. At the end of the document there are two additional sections. One with the benefits expressed by the researchers in our survey and another that comments on the gender perspective. Following OS values, this guide is under an open peer review process. If you have any comments or suggestions (regarding both the content and usability) please drop us an email at greco@greco-project.eu. The review process will go until November 2020, and the final version of the guide openly available in a new version in this same link.