I haven't personally used that board but I have built mods with other PCB boards. It's not that difficult, theres only 4 connections to solder (battery +ve, battery -ve) , (510 +ve, 510 -ve). Maximum input voltage will be 4.2 so single battery is no problem , dual batteries will have to be wired in parallel. Fasttech sell a cloned version of the istick 30w pcb https://www.fasttech.com/products/3023/10010021/3120900. If you check out the discussion there..you'll see the wiring discussed.
but depending on the board and your soldering skills the switches can sometimes be wired so you can install the switches anywhere in the enclosure. Example (scroll down a wee bit)
Good advice above, said it all really. Will be a simple build and a good entry into modding.....just when disassembling the istick.... don't snip the old cells pos and neg at the same time XD