Start by measuring and cutting all wires needed. It's good to keep them short since parts will be stacked. Remember, the Pro Trinket will be on the back of the OLED with the majority of holes being in close proximity. Allow extra room when you strip the wire ends so you will have adjustment room. You can always trim the excess off after soldering.
Prepare the JST connector cable by trimming off the male end of the connector. It won't be needed since you will be wiring directly to the Pro Trinket. Split the red and black wires for a short distance and then strip the ends so they are ready to go.
Okay, solder time. Make sure you tin your wires as you go. First solder wires on the Pro Trinket, remembering they will reach straight down to meet the back side of the OLED. It helps to use a third hand tool or vice. Remember to allow for two wires to be soldered together into the G (ground) on Pro Trinket.
Now, gather your patience. You will need to line up the wires from the Pro Trinket with the holes of the OLED. You want the Pro Trinket board's top and bottom edges to line up with the edges of the OLED, so the holes will not line up perfectly, but they will be close. This will require the use of a third hand tool, vise and tweezers. If you can't get them all to line up at once, then just work by soldering one wire at a time.
Once the solder cools, trim excess wire tips. Also, use a hot glue gun and add a pillow of glue inside the bottom edge of the Trinket. It should fall right below the reset button. Quickly push the Trinket against the glue. This will help support your cameo sandwich. While you are at it, swirl your battery wires against the right side of the OLED board near the Trinket and glue them in place for strain relief.