For normal numbers like 3.14159, the procedure is as follows:
separate the number into sign, biased exponent, and significand
add the difference in the exponent biases for long double and double
(0x3fff - 0x3ff) to the exponent.
assemble the sign, new exponent, and significand (remembering to make the
leading bit explicit in the Intel 80-bit format).
In practice, on common hardware with the Intel 80-bit format, the “conversion” is just a load instruction to the x87 stack (FLD). One rarely needs to muck around with the actual representation details, unless targeting a platform without hardware support.