Here you go. This is something I posted on another forums, so it is directly copied and pasted below.
Many people these days are trying their hand at planted tanks, and as such, sometimes CO2 injection becomes a necessity due to the high light they have. So, here's an invaluable piece of equipment that can be used to estimate the amount of CO2 in your aquarium.
Here's the equipment you will need, or rather, the stuff that I had lying around my house that I used:
1 Glass container with screw on lid
1 Silicon glue applicator/similar type glue applicator
1 Rubber O ring that is about the same size as the screw on lid
Power drill with appropriate drill bits
1 Suction cup
1 Zip tie
Pictures!

Glass container with lid

Glue applicator with the tip cut off so it will fit into the glass container

O Ring
As mentioned, you'll have to cut the plastic applicator so that it fits within your container first. Next, you'll want to drill a hole in the cap.
Finally, you can put the O-ring onto the applicator, put the applicator in, and screw on the lid.
Next, I needed some way to mount the whole thing in my nano, so I used a spare suction cup and a plastic zip tie.
You'll notice the zip tie just goes through the suction cup (i.e. I just put a hole through the nipple of the suction cup).
After the mechanical work was done, I had to start the chemistry part (I love chemistry!)
Items you'll need to make a 4 dkH reference solution for the drop checker:
1 Graduated Cylinder
1 Scale that can measure to at least 0.1 gram accuracy
1 Box of Baking Soda
1 Bottle of Distilled water
Pictures!

Scale that I bought off a site, it was $11 (free shipping)

Baking soda (sodium bicarbonate), you can pick this up at your grocery store.

Distilled water, again, grocery store.
First, since a 4 dkH reference solution is needed, I made the necessary calculations and prepared a 40 dkH solution before diluting it 10 fold to 4 dkH.
To make a 40 dkH solution, you need 1.2 grams of baking soda per 1 L of water. I made up a 2 L batch, so I needed 2.4 grams (the bigger the batch, the more accurate your final solution will become).
Then, taking 50 mL of the 40 dkH solution (measured with a graduated cylinder), I poured it into a bottle and filled the last 450 mL with distilled water (giving me a 1:10 dilution = 4 dkH solution).
After you make your 4 dkH solution, measure out 5 mL, and using a pH test kit (i.e. I used the API pH test kit, any test kit that uses bromothymol blue will be fine for this purpose), put in at least double the recommended number of drops (i.e. API recommends 6, I put in 8-9 drops). This makes the resulting solution darker, and easier to read.

Here's the liquid inside the drop checker.

Everything put together, just awaiting me to strap the drop checker to the suction cup.

The whole contraption has been placed into my nano, around 4:48 pm. At the time of writing this up (5:35 pm) the top portion of the liquid is already starting to turn green.

6:28 pm now, it's definitely green
