In order to achieve a step-down (5 V → 3.3 V) and step-up (3.3 V → 5 V) level shift (voltage conversion), how should this be achieved?

The following is a brief example.
For detailed selection methods, please use the following application notes.
→　Tips for Selecting Level Shifters(Voltage Translation ICs) (PDF:2.62MB)

Step-down voltage translation:
The input-tolerant function allows level shifting from a higher voltage to a lower voltage such as from 5 V to 3.3 V. For example, the input-tolerant function allows 5 V to be applied to an input without drawing extra current even when V_CC = 0 V.（Fig. 1）

For step-down translation using the input-tolerant function of a CMOS logic IC, ensure that the maximum low-level input voltage is within the V_IL range. (Table-1)

Table-1 Example of input specifications of a typical CMOS logic IC with a non-TTL input (74VHC240)

Step-up voltage translation:
Level shifting from a lower voltage to a higher voltage such as from 3.3 V to 5 V can be performed if the high-level output voltage (V_OH) of the preceding IC satisfies the high-level input voltage (V_IH) of level translation IC because V_OH is almost equal to the supply voltage (V_CC).
Since the minimum high-level input voltage (V_IH) of typical CMOS logic ICs is specified as V_CC × 0.7, V_IH is 3.5 V when V_CC = 5 V. Therefore, if the V_OH of the preceding IC is 3.3 V, the minimum V_IH of level translation IC is not satisfied. In this case, consider using a CMOS logic IC with a TTL input. (Fig. 2)

Table-2 Example of TTL input specifications of a CMOS logic IC (74VHCT240)

The 74VHCT240 cannot be used if the supply voltage of the preceding IC is lower than 2 V. In that case, it is necessary to use a level shifter (bus switch type: TC7MPB9326) as shown in Fig. 3 and Table-3.

Table-3 Supply voltage range of a level shifter (TC7MPB9326)

There is also a method of Step-up voltage translation using a product with open drain output. Due to the pull-up resistor R_pull, the output swings to the V_CC level. (Fig. 4)