Help selecting feedback and ground resistors for a gain and offset circuit
Let's analyze what you need to do.

1) Input voltage is plus minus 3.3V, reference supply is 3.3V.  Your choice of resistors R1 &amp; R2 is good.  Check: if input is 3.3V, input to op amp is 3.3V.  If input is zero Volts, input to op-amp is 3.3V /2.  If input is -3.3V, input to op-amp is zero Volts.

2) Determine the total gain of the input resistor network.  Inspection tells us that it is 0.5.

3) Determine the gain required from the op-amp.

I'm going to digress a little bit here.  The LM358 is an okay op-a,p. but lousy input offset voltage error and a major problem here: the highest output voltage is about (Vdd - 2 Volts).  IF you run the LM358 from a higher voltage than your 3.3V rail, you can make it work.  But if all you have is that 3.3V rail, you need a different op-amp.

Now let's look at what gain you need from the op-amp.

From statement (1) above, the output voltage range of the input divider network 3.3V through 0V.  Since that is also the desired output voltage range, the op-amp need a gain of (1.0) or unity gain.

It's really that simple!