BETYdb is designed for both previously published data and 'primary' data. Most of this documentation assumes that you have already identified a data set that you want to upload, or have a set of papers from which you would like to extract data and summary statistics.
If you are planning to do a meta-analysis, even if this is not your first time, please read 'Uses and Misuses of Meta-analysis in Ecology" \cite{Koricheva_2014}. Many texts are available, but the recent "Handbook of Meta-analysis in Ecology and Evolution" is probably the most comprehensive and specific for plant sciences.
For a meta-analysis, the first step is to find papers that contain the target data.
The easiest approach to use a search engine such as Web of Science{target="_blank"}, Google Scholar{target="_blank"}, or Microsoft Academic Search{target="_blank"}. Starting with queries such as "scientific name + trait", and allowing these results to guide further queries. Often, the references (particularly of meta-analyses and reviews) and forward citations will point to other studies.
Another starting point for the programmatically inclined - which aids in documenting searches - is to submit queries programmatically. Carl Davidson wrote a python script{target="_blank"} to search for citations based on species and trait name. In addition, the rOpenSci project has a suite of R packages for searching publications{target="_blank"}.