documentclass{article}

usepackage{fontspec}

setmainfont[Mapping=tex-text,Ligatures=TeX,Scale=1.05]{Times New Roman}

begin{document}

Ligatures: fi fl ffl

end{document}

; TECkit mapping for TeX input conventions <-> Unicode characters

LHSName "TeX-text"
RHSName "UNICODE"

pass(Unicode)

; ligatures from Knuth's original CMR fonts
U+002D U+002D           <>  U+2013  ; -- -> en dash
U+002D U+002D U+002D    <>  U+2014  ; --- -> em dash

U+0027          <>  U+2019  ; ' -> right single quote
U+0027 U+0027   <>  U+201D  ; '' -> right double quote
U+0022           >  U+201D  ; " -> right double quote

U+0060          <>  U+2018  ; ` -> left single quote
U+0060 U+0060   <>  U+201C  ; `` -> left double quote

U+0021 U+0060   <>  U+00A1  ; !` -> inverted exclam
U+003F U+0060   <>  U+00BF  ; ?` -> inverted question

; additions supported in T1 encoding
U+002C U+002C   <>  U+201E  ; ,, -> DOUBLE LOW-9 QUOTATION MARK
U+003C U+003C   <>  U+00AB  ; << -> LEFT POINTING GUILLEMET
U+003E U+003E   <>  U+00BB  ; >> -> RIGHT POINTING GUILLEMET

; additions for f ligatures
;U+0066 U+0066 U+0069 <> U+FB03 ; LATIN SMALL LIGATURE FFI
;U+0066 U+0066 U+006C <> U+FB04 ; LATIN SMALL LIGATURE FFL
;U+0066 U+0066        <> U+FB00 ; LATIN SMALL LIGATURE FF
U+0066 U+0069        <> U+FB01 ; LATIN SMALL LIGATURE FI
U+0066 U+006C        <> U+FB02 ; LATIN SMALL LIGATURE FL

teckit_compile rijke.map

documentclass{article}

usepackage{fontspec}

setmainfont[Mapping=rijke,Scale=1.05]{Times New Roman}

begin{document}

Ligatures: ff ffi ffl fi fl

end{document}

documentclass{article}
usepackage{fontspec}
setmainfont{Times New Roman}
begin{document}
verb|Default:                  |
fi fj fl ff ffi ffj ffl Th

verb|Ligatures=Discretionary:  |
{fontspec{Times New Roman}[Ligatures=Discretionary] % or `Ligatures=Rare`
fi fj fl ff ffi ffj ffl Th}
end{document}