One switch is ON/OFF, the other is BLINK/SOLID.  In the above photo in step 9, the ON/OFF switch is on the left and in the OFF position.  The BLINK/SOLID switch is on the bottom right and is in the BLINK position.

Insert the battery, positive side facing up.  Turn the ON/OFF switch to ON.  If you've done everything right, your LEDs should blink in about a twice per second heartbeat-style pattern!

One of the LEDs don't light up:

• Check the direction of your LEDs!  Did you solder them on in the proper orientation?  If you need to change the direction of an LED, you can do this without damaging the LED if you're careful.  Put your soldering iron to the side of the LED and add a bunch of solder until it blobs and touches both ends of the LED, and gently wipe the LED right off the board.  If your LED is still on the solder blob on the tip of your iron, immediately whack the palm of your hand holding the iron against the table.  This will cause the solder and the LED to drop off the tip, onto your table.  Gently wipe most of the excess off of your LED and try again!  We also give you an extra LED.

All of the LEDs don't light up:

• Check the direction of your LEDs per above.
• Check the direction of your battery.
• Check the direction of your 555 timer chips.
• Check the direction of your mini-MELF diodes.

The LEDs blink, but in a weird/wrong pattern:

• First, take your battery out and make sure your circuit board is clean, per the above step.  Try it again.  If it still is weird, go onto the following steps.
• The first one minute may in fact have a slightly different pattern, there can be a small "break-in" period.  (It's an interesting effect due the initial voltage & internal resistance of a fresh battery).  Just wait and see if it changes.
• Check the direction of your timer chips and mini-MELF diodes.
• Check your resistors for the correct values in the correct places.  You can look at the numbers written on top under magnification, and you can also turn the circuit OFF, and measure each one's resistance with a multi-meter.
• Check your capacitors for the correct values in the correct places.  The two most important ones are the 2.2uF C4 and the 10uF C1.  There's a difference in the sizes of those capacitors, C1 should be the biggest/tallest one on the board.  C4 should be the second biggest.  Double-check the assembly drawing.  You can also measure the capacitance in-circuit if you have a multimeter, but note you'll get slightly different values than "ideal" out-of-circuit values.  You should still be able to verify that they're in the correct positions.