When buy a secondhand oak front door that is 1930x840 and I want to fit it into a 2100x970 opening. How should I do this?
I'm not an expert, but my understanding is that once you have built a solid and strong door frame, or had a pre-built frame installed, your opening should be 2 inches wider (horizontally) and 1.5 inches taller (vertically) than your door. In other words you will have to install a door frame for your door that appropriately matches the specs on your door. Also, It seems you are using metric measurement units of centimeters (cm) in reference to your values for your dimensions. You may have to search a conversion chart that shows metric to standard conversions and provides the unique ratio between the different units of measure (mm, cm, inch, foot).
You will finished it. Come on