That page is mostly about adding WRAM to a discrete mapper. The SNROM board already has WRAM. OP wants to add a battery backup circuit to the SNROM board. It's two diodes and two resistors that are populated at the factory on SNROM boards with a battery but not on boards without one.
But that page does link to a
schematic that infiniteneslives posted. I imagine someone with more electronics knowledge than myself could take that schematic and the SNROM board photos at
NesCartDB's page for Spot, compare that to the photos at
NesCartDB's page for The Legend of Zelda (which uses SNROM with battery), and turn that into a recipe with a shopping list and placement instructions.