How To Trowel Concrete
Troweling concrete is a skill and there is no way around this. It takes practice to learn the muscle memory needed and soft touch that concrete working requires. The process of troweling starts in your dominant hand and proceeds by making cresent shaped sweeping motions in front of you all without lifting the trowel at all.
The pressure that you exert on the trowel must be perfectly even and remain level despite the fact that you are moving through a range of motion. Any variation in the angle or pressure of the trowel will result in crease lines or trowel lines in the concrete. If you are having trouble troweling without leaving marks then you may actually need to let the concrete set up a bit more before you continue troweling. Remember that you must trowel as little as possible to avoid raising water on the concrete.
Concrete Finishing Tools
The material that your trowel is made from is a direct indication of what it should be used for:
Wood trowels are used for rough floating and general shaping of concrete where a rough texture is desired. This would be common with a base coat for swimming pools or as a base layer for tile setting. The rough texture allows for maximum grab between multiple layers of concrete and these trowels generally have squared corners.
Aluminum trowels and magnesium trowels, sometimes referred to as mag floats, are used for rough shaping and initial pass for concrete where you will be following up with a steel trowel for a smooth finish. These trowels are also sometimes used for specialty mixes where the concrete tends to stick to the trowels. Similar to wood floats mag trowels also have squared corners.
Steel trowels are the backbone of smooth concrete surfaces. Pretty much all forms of concrete finishing will require a steel trowel as it allows for the lowest friction contact with the cement creating a smooth and continuous layer. Available in both square corners and also rounded corners where the latter makes is easier to trowel surfaces that change plane of existence and square corner trowels are generally used for flat concrete finishing.
There are also plastic and metal trowels formed to meet every possible degree of inside and outside surface angles which are used in detailed concrete applications and are affectionately referred to an "monkeys". Texture skins are used to add a decorative texture to smooth concrete surfaces and a regular bristled broom is all that is required to produce the de facto standard for cement finishes - the broomed concrete finish.
Concrete Finishing Techniques
Before you even think of picking up a steel trowel you need to screed the concrete to the exact levels that you are looking for. This process is usually completed by adding extra material and scraping down to get the ideal levels. Beginners have a tendency to make small adjustments, should the concrete level be too low to screed, where an expert would add a large amount of concrete to the low area and then screed it back off.
After the initial screed it is OK to lightly pass over the rough concrete with a steel trowel which will help greatly with finishing the concrete smooth later. If you choose to use a steel trowel at this stage you must be extremely light and steady handed to avoid leaving trowel lines in the very fresh concrete.
Alternatively you can mist water sparingly onto rough cement that has just started to turn, as well as using a sponge float or regular sponge to churn up some slurry on the surface. This technique of making small circular patterns with a sponge can help to achieve a perfect level to the concrete surface. Also the slurry that this process creates on the surface of the concrete is easily troweled smooth.