The FAR says:
(2) Forty hours of actual or simulated instrument time in the areas of operation listed in paragraph (c) of this section, of which 15 hours must have been received from an authorized instructor who holds an instrument-airplane rating, and the instrument time includes:
So 15 hours must be with an instructor. What about the other 25? Can I fly in instrument meteorological conditions (IMC) "solo" (not the hood) like I did for my private pilot license (PPL) after an instructor sign off? Or does this just mean that I need another instrument rated (IR) pilot with me, but it doesn't necessarily have to be a certified flight instructor (CFI)? If that is the case, and I use the hood rather than actual IMC, does the other pilot need an IR or just a PPL?
Basically, how do you fulfill the other 25 hours?