While battling a Temtem, instead of choosing a Temtem technique, go to your bag and select the TemCard, and then choose which Temtem you want it to be used on.
To increase your chance of capturing the Temtem, battle with it for a bit and make it weaker by dealing some damage or inflicting a status effect; such as Sleep or Freeze.
Do note that the damage you deal to it before taming will have to be healed before you battle with it. Each Temtem has its own capture chance, which varies according to the Temtem’s rareness.
Once you tame a wild Temtem using a TemCard, it will get put in your Squad. If your Squad already has 6 Temtem in it, the newly tamed Temtem will get added to the terminal. You can use a Temporium to view the terminal.