How to transliterate and pronounce English letters in Chinese?
For the transliteration of each letter into Mandarin Chinese, just follow the list below, written in pinyin with Chinese characters where possible. Some pinyin combinations may not exist in Mandarin at all though, so I will give approximate IPA or English transliterations.

A : ei 诶 /ei/ A
B : bi 比 /pi/
C*: xi 西 /ɕi/ or /si/ || "sei" /sei/ say
D*: di 弟 /ti/ || dei /tei/ 
E : yi 一 /ji/
F : "e-fu" /efu/ where the "u" is often only pronounced lightly producing a pronunciation of approximately /ef/ as pronounced in English. Note that the "fu" is most often pronounced with the neutral(zeroth/fifth/light) tone
G : ji 计/鸡 /t͡si/ similar to Z
H : ei-chi /eit͡sʰɨ/ where the "ɨ" is often only pronounced lightly producing a pronunciation of approximately /eit͡sʰ/ "age" as pronounced in English
I : ai 哀 /ai/ I
J : "zhei" /t͡sei/
K : "kei" /kʰei/ K
L : /el/ L note that this letter is most often pronounced with the second(rising) tone
M : em /em/ M
N : en 恩 /ən/
O : ou 欧 /oʊ/
P*: pi 批 /pʰi/ || pei 胚 /pʰei/ pay
Q : "ki-ou" /kʰiou/
R : /aɻ/ R note that this letter is most often pronounced with the second(rising) tone
S : "e-si" /esɨ/ where the "i" is often only pronounced lightly producing a pronunciation of approximately /es/ "as" as pronounced in English
T*: ti 梯 /tʰi/ || "tei" /tʰei/
U : you 优 /joʊ/ yo
V : wei 微 /wei/ way
W : da-bo-liu 大播柳 /tapuo'lioʊ/ I do not know how to describe the tone for this.
X : "eg-si" /eksɨ/ where the "i" is often only pronounced lightly producing a pronunciation of approximately /eks/ "ax" as pronounced in English
Y : wai 歪 /wai/
Z : ji 计/鸡 /t͡si/ similar to G



Except where noted, the tone is either the first(high) tone or the fourth(falling) tone.


Indeed many words are transliterated from a small set of Chinese characters. However, there is not rule for transliterating any word and thus all those you have seen are just a convention which can be learned by experience, i.e. you'll just have to read more of those words in order to guess a more probably (and never exactly) correct transliteration of a word. Here's a tip: technical fields aside, most transliterations choose more "pleasant" sounding characters, e.g. 德 "virtue"(positive) over 得 "obtain"(neutral) and 布 "cloth"(neutral) over 不 "no"(negative) and 怖 "terror"(negative).