This is not possible without additional hardware.
While you may find some articles that have only passive components in schematics, like this one: http://www.idiots.org.uk/vga_rgb_scart/ (sites like this warn you that all you do - you do at your own risk).
The both cables have a separate lines for the Red, Green, and Blue signals. But the synchronization signal is transferred differently in these two standards. VGA uses separate H-Sync and V-Sync signals while Scart has only one composite synchronization signal. Here you may see the schematic of a simple converter that mixes up H-Sync and V-Sync of the VGA cable into the Video sync signal. However I would propose you to find a ready to use solution