You
need to figure out your planet's year length as it relates to the
earth year.
If
your planet's year is in earth days right now
(Example: My planet's
year is 112 earth days long) you need to do this:
My
Planet's year in earth days =
365

112
365 
=
.3068493 
Throw
away most of those decimal numbers: your planet's year = .3
earth years
(about a third of an earth year.
That makes sense)
However,
if you already have a number that is about earth years
(Example: My planet's year is 4.7 earth years long) then
use that number as it is.
Use
Kepler's formula:
This is the Formula 
(this planet's year in earth years) x
(this planet's year in earth years) 
= 
(distance from sun in AUs) x
(distance from sun in AUs) x
(distance from sun in AUs)

squares
of the sidereal periods 
= 
cubes
of their semimajor axes 
(sidereal
periods)^{2} 
= 
(semimajor
axes)^{3} 
Let's do a couple of examples.
Example 1:
My planet's year is .3 earth years
long.
Find
the distance from the sun of a planet with a year that is
0.3 earth years long. 
squares
of the sidereal periods 
= 
cubes
of their semimajor axes 
(this
planet's year =0.3) x
(this planet's year =0.3) 
= 
(distance
from sun) x
(distance from sun) x
(distance from sun) 
0.3
x 0.3
Use Calculator: 0.3 ^ 2 =

= 
(distance from sun) x
(distance from sun) x
(distance from sun) 
0.09 
= 
(distance
from sun) x
(distance from sun) x
(distance from sun) 
So
. . . this planet's
AU 
= 
the
cube root of 0.09 
Type this on the calculator keyboard: 0.09 ^ .33 =
Your
answer should be 0.45175194192142615
(You
don't need all these decimals)
The planet's
distance from the sun is about 0.45 AUs
Your
planet is about half as far from its sun as the earth is from
Sol.
Example
2: My planet's year is 3.7 earth years long
Returning
to Kepler's Law:
This
is the Formula 
(this
planet's year in earth years) x
(this planet's year in earth years)

= 
(distance
from sun in AUs) x (distance from sun in AUs) x (distance from
sun in AUs) 
squares
of the sidereal periods 
= 
cubes
of their semimajor axes 
(sidereal
periods)^{2} 
= 
(semimajor
axes)^{3} 
Find
the distance from the sun of a planet with a year that is
3.7 earth years long. 
squares
of the sidereal periods 
= 
cubes
of their semimajor axes 
(this
planet's year = 3.7) x
(this planet's year =3.7) 
= 
(distance
from sun) x
(distance from sun) x
(distance from sun) 
3.7
x 3.7
Use Calculator: 3.7 ^ 2 =

= 
(distance
from sun) x
(distance from sun) x
(distance from sun) 
13.69 
= 
(distance
from sun) x
(distance from sun) x
(distance from sun) 
So
. . . this planet's
AU 
= 
the
cube root of 13.69 or 