First, s/he wants to learn patterns, or motifs in chess term. The more motifs s/he knows, the faster s/he can generate ideas over the board. In chess games, especially for beginngers, ideas are much more important than calculation. If s/he can come up good ideas often, s/he will win more games. Without ideas, calculation is just brutal force and will not be efficient.
Second, s/he learns how to calculate. Most problems should be 2-5 moves deep, with a few variations. That's the type of problems s/he will face in his/her games. S/he should sense the possibility for a tactics, see it, calm down, and calculate all variations. Because during the tournament games, time is limited. S/he has to learn to quickly decide how much s/he can allocate to find solution. Time management is critical. when s/he has enough time, s/he should be patient, following the variation move by move, remembering everything, clarifying each line, quickly confirming or denying any short variation, and concentrating on one or two long variations.
Third, s/he learns to double or triple check his/her work. S/he needs go over all possibilities to search for any hole in his/her solution. Sometimes s/he has to redo his/her calculation.
Through training, s/he should refine these processes, making them more automatic, smooth, and efficient. In the end, s/he should have higher confidence that s/he has found the correct solution, and most times it should be true.