Shiva Sutras by Jaideva Singh, Section II(comprises of 10 Sutras) greatly deals with mantras, mudras and science, etymology behind it.
The very first Sutras says:
चित्तम् in this context means that by which the Highest
Reality is cognised.
मन्त्रः a formula consisting of a word or a set of words addressed to a deity.
"By intensive awareness of one's identity with the Highest
Reality enshrined in a mantra and thus becoming identical with
that Reality the mind itself becomes mantra."
From the commentary of first Sutra, few terms explained further in Note section:
prasāda and praṇava;
prasāda which is a technical word of this system which will not
be found in any dictionary. It is the name of the mantra sauh(सौः). It contains within itself the entire panorama of manifestation. It represents all
the 36 tattvas.
praṇava is the mystic sacred syllable. According to
Saivagama, there are four kinds of pranava,
(1) the Saiva Pranava which is हुं(huṃ)
(2) the Sakta pranava which is क्लीं(klīṃ)
(3) the maya pranava which is ह्रीं(hrīṃ) and
(4) the Vedic pranava which is ॐ(oṃ)
The pranava referred to in this sutra is the Saiva-pranava.
From same book, Page 89: As has been said in Tantrasadbhava:
"O dear one, all mantras consist of letters. The letters are a
form of sakti. That sakti should be known as matrka. Matrka
should be known as the very form of Siva."
From the same book, page 111, a table described the letters with associated shaktis and tattvas:
I cherry picked some of the content from the book, So I would suggest to read this section II, that will give understanding how mantras and beejakshara are formed. Though this section describe it very well, but it is advised to receive this knowledge from a Guru himself.
Also to know the history of Shiva Sutras, refer this answer: What is the story of revelation of Shiva sutras to sage Sri Vasugupta?