Hold [Vol Up]+[Home]+[Pwr] buttons until phone reboots (i9100 splash screen).
EDIT: OK re-read, does your recovery have an option to mount USB?
EDIT2: If not, Philz kernels have the best USB mount function. Download the Odin flashable version here. Boot into Philz recovery, mount filesystems over USB, copy (1) GS2ROMNuke (2) CM10.1 (3) gapps .zips onto phone, then install (in sequence).
BTW CM10.1 install will overwrite the Philz kernel with its own (you'll just use it temporarily).
I was worried about the battery mostly. So I installed jellybam again and charged. I was not sure if it would charge in recovery/download mode, thanks CodeMaster for pointing that out to me. Gonna try the nuke and flash now.