First, you need to go through the process of cleaning O2. Inspect your tank on a daily basis, so you know that you are not breathing in from unclean tanks. You don’t have to be a professional tank inspector to know whether the tank is unclean or not, just open up the screws and start checking. While filling the tank with O2, make sure you fill it slowly, or you will end up having a fire.
In order to start mixing your gases, you will require a bit of plumbing. Use a gauge to check whether the amount of gas you have filled in the tank is the same as you require.
You must drain your tank empty before adding any gases together. Attach the tank to your helium tank and fill in with the required pressure. After you are done, repeat the same step with the O2 tank. The reason why you add helium first is because you don’t have a lot of O2 in trimix anyway. In order to account for temperature, you must add a little extra helium. It’s a rule of thumb to keep the amount of oxygen low as compared to helium inside the tank.
Test your mix once you have filled these two gases. It might be a little dangerous if you keep inhaling O2, so make sure that you hang out for a bit and test whether you are feeling okay. Try not to inhale too much at the same time, as it is not good for your health.
Top your tank off with a bit of air. You must go to your local dive shop in order to fill in air. Explain to them how you don’t want to drain your tanks at all and want to fill in air with the exact amount of pressure that is already in the tank.