SCART is RGBS+ground. Red, Green, Blue, Composite-Sync, and a ground connection. Composite video or Luma will often do for sync as they each contain video information over composite-sync. VGA is RGBHV+ground. Red, Green, Blue, H-sync, V-sync, and a ground connection.
I can't tell if the JPac will take an RGBS signal, there's nothing saying it will. Which means it needs RGBHV, which is what a PC outputs for analogue video. You can use an LM1881 chip to split sync from a console, just feed composite sync or luma in, if that's all you have, instead of composite video. You get out vertical sync and composite sync, and you use the latter where you need horizontal sync on VGA. This is assuming the JPac will like it. Most things do, but without confirming you're taking the risk it won't work right. You can ask Andy from Ultimarc, he as a rep for being really helpful.
Google the pinouts for VGA and SCART. Then google more, and compare. Don't mistake JP21 for SCART, and make sure you have an up to date version of VGA. Wikipedia is ok for this stuff, but tends not to have great pictures.
Then you look at those pinouts and work out what goes to what, red to red, blue to blue, etc. Use 75ohm coax where possible, and connect the screening to ground. Most often the ground lines are all connected together at each end inside the PC or monitor, but test this with your multimeter to confirm.
This excludes audio, which you will have to break out separately, as the J-PAC won't take it via the VGA D-SUB plug.