First of all, read EML's article to make sure you're comfortable with the electronics. I had a lot of issues getting the 555 timer to reliably trigger the 4017, so used the 9v LED flasher circuit from Bill Bowden's site which worked much better (for me). Use the following component values: R1=10K, R2=100K, C1=0.47uF
I built my circuit on two pieces of stripboard:
1) A long piece with 3 LEDs per channel plus the resistor, capacitor, transistor circuitry than gives the LEDs their 'afterglow' effect.
2) A small piece with the 555 and 4017 which was shaped to fit in an Altoids tin along with a 9V battery.
Step 1: Prepare a Plastic Tube
1) One large central hole to thread the wires through. Use your biggest drill for this and make the hole even larger with a knife if necessary.
2) Two smaller holes that wil be used to mount to the Altoids tin. Use your smallest drill for these - the screws will make their own way. They should be about one inch either side of the large hole.
Step 2: Mount the LEDs in the Plastic Tube
The inner diameter of my tube was a bit smaller than the finished circuit so I had to squeeze it flat to get it in. This has the fortunate side effect of holding it in place.
Caution and care is needed at this stage to ensure the wires don't get caught up.
Step 3: Altoids Tin
I drilled one hole on the side to mount an on-off switch, and four on the bottom through which I threaded cable ties (to attach to the handlebars).
Step 4: Final Assembly
Mount it onto the handlebars using zip ties and enjoy the adoration of every other two year old in the playground. At least, you will if they are still there when it gets dark...