I'm not sure what you mean by "procedure." Just gather the parts, solder them together and test it. The schematic doesn't give much help in estimating how heavy the wiring should be for the power (12Volt) and ground connections to the two transistors, or how big a heatsink they will need, or even how big the transformer is. The transistors' maximum power and current capacity will determine the maximum size of much of that, but it would be prudent to operate it at less than half that, particularly if you are unfamiliar with power circuits.
The output of this inverter will be a square wave, which is less efficient (and possibly damaging to some equipment) than a power-line sine wave. The "freq" adjustment presumably can be set for 50 or 60 Hz (or anything else!), depending on what you need. I'd guess you can expect a conversion efficiency of 50% or so with this circuit; that is, for, say, 25 watts at 240V output, you would need to supply about 50 watts at 12V DC input (4.2 Amperes).
As Mr. Harrington has said, there are better circuits to do this nowadays, but this is a nice simple hobbyist arrangement to play with. But watch out for the dangerous high voltage!
