There are 4 versions for the back half of the case:
Those marked as XL are 3mm bigger to accommodate a 1800mAh battery like this one.
The versions marked "Support" have an extra pillar in the center that will make it feel more rigid if you aren't using the space for other modifications.

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There are 2 versions for the face buttons:
Those marked "soft" are for the rubber membranes included in the kit.
Those marked "click" are for use with tactile buttons like the Start/Select buttons.

To print the standard case, you will need to print the following STLs:

1. Null2 Back V1.2.STL

2. Null2 Bottom Button V1.2.STL

3. Null2 Dpad Soft V1.2.STL

4. Null2 Face Buttons Click V1.2.STL

5. Null2 Face Buttons Soft V1.2.STL

6. Null2 Front V1.3.STL

7. Null2 Mute Switch V1.2.STL

8. Null2 Power Switch V1.2.STL

9. Null2 Shoulder Buttons V1.2.STL

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I recommend that you print the files at a 0.15 layer height or smaller.

Make sure your software imports the files at the correct scale. The case is 154 x 62 mm.
Print the front and back halves of the case with the large flat surface against the print bed.
For the screws, four M3 x 8mm countersunk screws like these should be fine.

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There's now a version available that works with DS Lite buttons thanks to woozer: