I started out using Paul’s generic panel, as @Bloodbat already linked to, and worked out from that (started to add input/output-ports and knobs). Browsing from the VCV library you can find links to multiple GitHub project so you can see how others are building panels (e.g. how ports/knobs are added, and used).
I was already proficient using SVG, when I started out, however the Developer information provided described the size of panels in mm, and I couldn’t get it working. So the link below is the reply I got as how to scale the SVG panels using pixel instead (15 pixel for each HP in width, and 380 pixel high):