Connect all the grounds, ideally from each board back to the battery. Use voltage level shifting chips to convert from 3.3V to 5V, and from 5V to 3.3V.
3.3V into 5V may be okay as is. If not, buffer with a 5V powered device, such as 74HC125.
cd74HC4050 works great for 5V to 3.3V.
Both available at digikey.com in thru hole and SMD variants.