I have a project with the CRS set to EPSG:31370. In this project there are a couple of layers, with a different CRS than the project CRS namely EPSG:4326 and EPSG:31370. Now I need to calculate the area of the polygons in those layers in square meters. However, I can't seem to find how I can calculate the area in square meters when using a shapefile with the EPSG:4326. I've already googled, read the corresponding data in the manual, youtubed and did a search on this StackExchange, and while I'm convinced my problem is an easy one to solve, I can't seem to find the solution.
What steps should I take to be able to calculate the area of shapefiles with CRS's that have degrees as their units?
I am quite new to GIS.