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According to Wikipedia, in Class G airspace, "Operations may be conducted under IFR or VFR. ATC has no authority but VFR minimums are to be known by pilots. Traffic Information may be given as far as is practical in respect of other flights."

How do IFR flights in IMC in Class G airspace not collide with each other?

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IFR airplanes are expected to coordinate with each other if two airplanes arrive to shoot an approach at the same time in uncontrolled airspace. Whomever arrives first is going to be announcing their intentions per the reporting requirements, and someone following, if they are that close behind and could have a conflict, will have to figure out an appropriate way to delay their arrival, such as holding at a fix. There's nobody to tell you what to do, so you just use common sense.

The key thing is the requirements are to report your arrival, and your progress at specific stages in the approach, and if you are sharing airspace with another IFR flight arriving at the same time (pretty rare), you coordinate your actions to stay out of each other's way in the same way you would if you were VFR.

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    $\begingroup$ What frequency is used? The CTAF? What if they're in cruise flight, not shooting an approach? $\endgroup$
    – Someone
    Mar 25 at 2:10
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    $\begingroup$ An airport in uncontrolled airspace that has an instrument approach will have a Mandatory Frequency that is to be used for all flights. The MF will be shown on the approach chart. It may be a unique frequency, or it may be a Unicom or regional Aeradio frequency. If an airport has an MF you must broadcast your intentions whether VFR or IFR. $\endgroup$
    – John K
    Mar 25 at 3:06
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    $\begingroup$ You seem to be answering the question with a USA-centric bent. In the USA it is exceedingly rare for Class G to extend higher than 1200' AGL, which all-but-precludes IMC flight, per 14 CFR 91.119(a); thus the most common scenario for two IFR aircraft to meet in Class G is at an airport. In other parts of the world Class G is much more extensive and @Someone's question is not so easily answered. $\endgroup$
    – randomhead
    Mar 25 at 4:28
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    $\begingroup$ I'm in Canada, where uncontrolled IFR is common in the North. $\endgroup$
    – John K
    Mar 25 at 5:11
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    $\begingroup$ Yes you are allowed to operate NORDO at an MF airport. $\endgroup$
    – John K
    Jul 8 at 0:14
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Ideally, all aircraft will be giving position reports on CTAF (which does exist for en route in some places) and will self-deconflict.

Where CTAF does not exist, or if pilots choose not to use it, then worst case we're relying on Big Sky Theory.

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    $\begingroup$ Does "big sky theory" mean "the sky is big, so there probably aren't two planes in the same place"? That doesn't sound like a good idea to rely on $\endgroup$
    – Someone
    Jul 7 at 16:09
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    $\begingroup$ @Someone-OnStrike That's exactly what it means, and unless you're low near an airport, it's surprisingly effective. The sky is very big, and planes are very small. $\endgroup$
    – StephenS
    Jul 7 at 16:16

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