cut, join, tape. DONE. (shown outside guitar for visibility - use *existing* clip & don't detach!!!)
Step 1: Split Existing 9v Clip
Get to the 9v clip, carefully remove rear sleeve, and cut in two without detaching or damaging either wire
Step 2: Attach 1 Half-clip to Each Battery & Join
They're sized differently so you can't mess up polarity, doing it backwards is impossible.
Step 3: Tape It Up
Just click em together and tape it all around to secure hold.
Step 4: ..and PROFIT!
PS fresh batteries read >9v each, that's OK. ~17-19v is good, if less replace your old battery. never use a drained battery with a fresh one. if no tester available, use two new batteries instead
Step 5: Reversal Info & 27v Option
27v is pushing it for EMGs, supposedly. do at your risk with fresh batteries, who knows if 28-29v could be harmful and zap the preamp....not sure