If the PCB is small and it moves easily, then it's recommended to clamp it in a small vice or in a PCB holder jig. It is expedient to start soldering with the smaller components, and continue with bigger and bigger components.
The solder iron must be heated to the temperature specified for the solder wire. For hobby work, people usually use leaded solder, it can do with lower soldering temperature, but it's not environmentally friendly. The soldering iron must be touching and heating the component lead and the PCB pad at the same time, otherwise an unreliable (cold) solder joint may occur. Do not overheat components. It depends on the temperature and the actual component type how long it can be heated before it's damaged. To be on the safe side, do not heat component leads for more than 3-4 seconds. If more time is needed, first let the component cool down, and retry soldering only after that. If you've heated the solder for too long, and the flux in the core of the solder wire has evaporated, then you must refill it (e.g. by dipping the tip of the solder iron into rosin flux), otherwise a non-wetting solder joint may occur. FYI the following article deals thoroughly how to repair common soldering problems: https://learn.adafruit.com/adafruit-guide-excellent-soldering/common-problems. After you finished soldering a component, check the solder joints visually.