So, you got source data model (graph in your case) and target data model (SQL in your case)
btw usually people do vice versa - they convert tabular data into graph in order to achieve advantages which graph model provide you with
First of all, you need to document both models for yourself. Than you need to design a mapping between the two.
Than you can use i.e. Talend open source data integration solution to implement your mapping in reality.
Other option is just export your source data with some scripts into .csv files which reflect target data model and import them into target DBMS with it's corresponding tools then.
WBR,
Andrii