The process of transcription proceeds as described here:
A transcription unit is located between the transcription initiation and termination sites. RNA polymerase begins transcription at the initiation site that is in a promoter. For example, in eukaryotes, RNA polymerase II spots the promoter through the presence of the TATA box, a region rich in thymine (T) and adenine (A) bases. Transcription factors can help RNA polymerases find transcription initiation sites.
The RNA polymerase separates the two strands of the DNA molecule and moves in the 3 'to 5' direction of the DNA or from 5 'to 3' on the premessing RNA.
In front of the coding strand of DNA, it associates nucleotides which are paired with those of DNA:
*G is facing C
*C is facing G
*A is facing T
*U (uracil) is against A (there's no thymin in mRNA but uracil instead).
During elongation, a premessing RNA is synthesized. The RNA polymerase stops at the transcription termination site (which its sequence depend on species).
After transcription, the processing of the premessing RNA will yield the mRNA that will be translated into protein in the cytoplasm.