240L is pretty big tank.
I would not do it the same way as 'Alto' suggested not that he is wrong OFC as he has far more experience than me
You have no livestock ATM so perfect time to setup the CO2. You dont need a pH pen but one is very handy to check your pH profile is stable once the lights are on. But you could start using the CO2 but if the [CO2] is fluctuating once the lights come on the plants will suffer if the [CO2]/pH fluctuates esp in the first 4-5hours. If you cant afford a decent pH pen I would go for some decent pH test strips but dont know which ones to advised as I have always used a pH pen/probe/controller.
You could go down the CO2 on 24/7 route as some do but it does use more CO2 OFC. But it does make things easier esp seeing the plants you have a dark green DC may be more than enough dependant on your light output
I have used a cheap pH pen is the past and did find it useful but not to measure the pH drop
per sa but just to measure if its stable and to ignore the actual reading value if that makes sense. So if it says says 7.10ph or 6.10pH it doesnt mater its the relative change it shows from lights on for first 4-5hours or till CO2 off. This method is subject to the errors the cheapo pH pens but with just justing the relative change after the Main pH drop IMO it minimises the errors in the actual pH reading. If going down this route still calibrate the pH pen but dont expect it to read the same valve day after day your just going of the relative change
If you have a high output light, with your CO2 equipment I would start the CO2 at a reasonable high BPS with the DC in the tank, the DC needs to be in a place where it doesnt catch any CO2 bubbles. Then allow 2-3hrs to allow the [CO2] to reach equilibrium in the tank then you need to wait 2 hrs for the DC to change colour and stay at that colour , I would aim for yellowish/green first then dial back a bit on the BPS esp if it goes yellow. After 2hrs of CO2 being on use the cheap pH pen and start measure the pH and see if it remains relatively stable for the rest of the CO2 period esp from lights on using paper and pen and make a note of time and pH as you soon forget when you last did it, do it every 30mins
Also we have very little details of your tank like filter output lights etc and a FTS helps too. Plus the flow/turnover in the tank needs to be good esp with a 240l tank. Setting up CO2 on a tank is very hard to do right.
After writing all that I think the method 'Alto suggested would be the best bet as small steps as a beginner to CO2 will help you understand whats happening which will help you get the CO2 right
But try using the cheapo pH pen after lights on as it may help