Here, I will discuss how to put everything together. Hopefully, I will have some instructive images as well.
The setup I will be describing is as below:
The general design is as follows:
CO2 tank -> CO2 Regulator -> Solenoid -> Needle valve -> Bubble Counter -> Diffuser
Some points to watch out for:
Between the CO2 tank and the CO2 regulator, you will require a washer to prevent leaks. This can be a disposable (one time use) nylon washer, or a perma-seal. Sometimes, you can get a nylon washer for free when you get your CO2 tank refilled (just ask!)
Between the regulator and the solenoid: Ensure that the fittings match (i.e. 1/8" to 1/8", or get the necessary adapters. Adapters can be bought for a few dollars at Home Depot and/or Rona.
Between the solenoid and the needle valve: Again, ensure that the fitting sizes match. If they don't, you can buy the necessary adapters at the hardware store.
Between the needle valve and the bubble counter: Again, the same rule as above applies.
Finally, for all connections: You will require either teflon tape or pipe compound (sometimes called pipe dope) to ensure that all the connections are well sealed. I do not recommend teflon tape before the solenoid as there have been reports of people getting small bits of teflon tape lodged within their solenoid, preventing it from closing all the way (this means CO2 will still flow!). As such, I recommend pipe compound instead (this can be bought from the hardware store for a few dollars as well). Use it sparingly, and don't use it excessively.
After putting the entire setup together, be sure to check all the connections for leaks using soapy water. By brushing/spraying the soapy water onto the connections, you will be able to detect any leaks due to the formation of tiny bubbles.