When consonants are combined with (r) it is indicated by giving curve above consonant.
Consider English words “part”, “harp”, “mark” etc. In all these words sound of “r” is half. So transliterating these words in Gujarati they will be written by giving curve above consonant which comes after “r”
Part :- પાર્ટ (pArT)
Harp :- હાર્પ (hArp)
Mark :- માર્ક (mArk)
When ર(r) is second consonant i.e. ર(r) is pronounced full but previous consonant is pronounced half, in that

Rule 1) Small slanting line is given below consonant to indicate succeeding full ર(r)

e.g. in English word Prophet, “P” is pronounced half and “r” is full. So in Gujarati it is written as
Prophet :- પ્રોફેટ (propheT )
Cockroach :- કૉક્રોચ (k~okroch)
Other native Gujarati words
ભ્રમર bhramar)
ગ્રહણ grahaN )
તીવ્ર tIvr )
સહસ્ર sahasr)

Rule 2) This is variation of style 1. It is used with ટ (T), ઠ (Th), ડ (D), ઢ (Dh), છ (Ch)

Here an arrow like symbol is given below consonant that is pronounced half
Few English words will be transliterated as
Tram :- ટ્રામ ( TrAm )
Drum :- ડ્રમ ( Dram )
Driver :- ડ્રાયવ્હર ( DrAyavhar  )
With other  (Th),  (Dh),  (Ch) it will be seen as
ઠ્ર (Thr)  ઢ્ર (Dhr)  છ્ર (Chr)

Rule 3) This is another variation of style 1. For ત(t) and શ(sh)

a new symbol is used to indicate to indicate ત(t) or શ(sh) are half followed by ર(r)
ત + ર = ત્ર (tr)
શ + ર = શ્ર (shr)

Rule 4) When રુ(ru) or રૂ(rU) is added to any consonant

it is written by giving curve below consonant.

સૃ  (sru) , કૃ  (kru) , તૃ (tru)
સૃજન (srujan)
કૃષ્ણ kruShN)
તૃણ ( truN)