How can I build the 40 hours instrument time required for an instrument rating? 14 CFR 61.65(d)(2) 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) &quot;solo&quot; (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?
The remainder of the 25 hours can be completed with a Safety Pilot, a CFI, or a CFII. 

A safety pilot must be able to act as PIC of the aircraft you are flying. Also, if the safety pilot is acting/desinated as PIC, both you and the safety pilot can log PIC time for when you are flying under the hood. If you are acting as PIC, the safety pilot can log SIC time for being a required flight crew member (FAR61.51(f)).

You cannot file an IFR flight plan or fly in IMC unless a current Instrumented Rated pilot is on board and acts as PIC.