I made my own 4 dkh solution a few years ago when I was into planted tanks and Co2. If you want to make your own solution you need distilled water, baking soda and an accurate scale to measure the weight of the baking soda (sodium bicarbonate). You can find the recipe on the internet. A little bit of googling or on this forum would probably find it pretty quick.
In brief, there is this one:
You first add 6 grams of baking soda to one litre of distilled water to give you one litre of solution at 200 dkh. Then you take 10 ml of that solution and you dilute it into 490 ml of distilled water for a final solution of 500 ml at 4 dkh.
I remember when I did it I didn't have a scale so I researched on how much powder of baking soda in teaspon or half tsp or what have you, would equate to 6 grams based on the weight mass. Not very accurate but hey I did it anyhow. The good thing was that I also had some commercialy bought 4 dhk solution to compare with. I had 2 drop checkers in the tank, one with my solution and the other with the commercial solution. Both gave me the same results and pretty much the same green. Perhaps it was luck that my measurements worked. After having played around with plants and Co2 and all, I don't think it is crucial that the soultion be absolutely correct and that consequently you end up in the "ultimum" Co2 ppm concentration level..There are other factors besides Co2 level such as lighting, nutrients, plant species and quantity of plants in your tank.
Is this a necessary thing to have even with the nutrifin C02 canister with DIY mix (got it free so I have it lol). I only ask because I know pressurised is a lot more powerful whereas DIY isnt so much. XD
It all boils down to your overall tank setup and what you are trying to achieve. I would say that the more you get into the so called Hi-Tech approach, the more important the use of the Co2 drop checker and the pressurized Co2 system as opposed to DIY Co2. For anything bigger than 20-30 gallon, I probably wouldn't bother with DIY Co2 since it would be too bothersome to get any decent Co2 level and consistently. I started with DIY Co2 but I quickly became tired of having to prepare a new mix every 3 weeks top. Yeast go through different phases in the fermentation process with the sugar and the Co2 output during each of theses phases is inconsistent over time. Could be fine under a low tech or low light setup but not desirable under a high light/high Co2 demand setup.