Start with one route
Published walkthroughs and media guides describe multiple endings. Mixing requirements from several routes is the easiest way to create an ambiguous save.
Write the target route at the top of the playthrough and record the important customer choices as they happen.

Protect the final branch
Keep a save before the last decision point when the game permits it, but do not rely on a downloaded save whose earlier choices are unknown.
A clean personal checkpoint makes the result reproducible and preserves the story context.
- Choose target ending.
- List route decisions.
- Record completed customer arcs.
- Save before the final branch.
- Confirm the ending and related unlocks.
Recheck after patches
Post-launch updates can fix story or achievement behavior. Compare the current official notes before repeating a route built around a launch-week bug.
If a condition appears not to register, document the route and version before changing several earlier choices.

Sources
AllThings.How full-game walkthrough →