If you are starting with feet in decimal form, say A1 contains 12.1, then
=INT(A1)&"' "&INT(12*(A1-INT(A1)))&""" "&TEXT(MOD(12*(A1-INT(A1)),1),"##/##")
will display
12' 1" 1/5
as feet-inches-fractional inches
If you are starting out with feet and inches, then put them in separate cells.