Before DVDLab. As I say in my notes, extract your SVCD MPG file off your CD with Isobuster. You then load it into DVDLab and it asks to demux it, say yes. It then crerates the 2 files in question. It may also say the audio has the wrong sample rate (such as 44.1) and it MUST be converted to 48khz. Let it do that also. You can then just proceed by loading in more MPG files and let it convert them for you. Then mak your menu and do the final compile.
This adds in MP2 audio to the final DVD, thats not a standard for NTSC DVD's so it may/may not play back. Thats when you have a look for the extracted *.mp2 (or *.mpa) file ats been converted to 48khz by DVDLab and use ffMPEG to convert it to AC3, remove the mp2 from DVDLab and add the ac3 in its place.