A very solid approach is laid down in the Manual of Standard Tibetan in the Appendix 7.
For people who don’t have the book I can see they collected this in the The Tibetan & Himalayan Library under THL Simplified Phonetic Transcription of Standard Tibetan
Summary
The correspondence between Tibetan letters and Wylie letters is the exact same except for ཅ་ which is cha instead of ca, and འ which we write as a instead of ‘a.
Superscribed letters (ར་ ལ་ ས་) fall down and disappear during transcription. and so do prefixes (ག་ ད་ བ་ མ་ འ་) and silent suffixes (ས་ ད་).
But some superscribed letters or prefixes (specially when not at beginning of word) sometimes they can be pronounced so we transliterate them in, such as བློ་བཟང་ “Lobsang” or རྡོ་རྗེ་ “Dorje”. So that leads to the question: 33. Transcribing Tibetan Words When does the prefix letter sound? in Tibetan Language Questions Google Doc.
Vowels a, o, u followed by ད་ ན་ ལ་ ས་ acquire umlaut ä, ö, ü.
Note
I personally will do what many people commonly do to avoid umlaut in ä they approximate it to e, so instead of Jampäl they write Jampel or Thubten instead of Thubtan.
Suffixes ག་ བ་ get devoiced as k, p.
བ་ at second syllable of word is wa.
པྱ་ cha, ཕྱ cha, བྱ ja.
About ར་-attached clusters: tr, thr, dr.
Voiced radicals ག་ ཇ་ ད་ བ་ ཛ་ or a cluster གྱ་ བྱ་ without the prefix that causes aspiration loss is transcribed as gh, jh, dh, bha, dzh, ghy, jh.
དབྲ་ r, དབྱ་ y, དབ་ w.