You say "it's too complicated to test" but...

...I hear too complicated NOT to test.

If it's really that complicated, how can you ensure everything is correct?

If it's complicated, how can you be sure that you won't introduce unintended side effects (break something that works) when you make any future modifications?

If it's simple to do manually but would be difficult to automate, detailed manual instructions may be a suitable alternative. Just don't try to rely on remembering everything involved.

It may be difficult but would creating automated tests be really valuable and give you confidence that things work without a lot of repeated manual effort?