First of all you need to "root" your G1
Tutorial:
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Secondly, you need to install this custom rom
Link:
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After those steps are complete
Download the two files from this..
Link:
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The files are the "Enabler" and "Disabler"
Once those steps have been complete, follow the rest of the tutorial on the PSFREEDOM link.
After this, if you dont know where to go from there follow this..
Link:
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If you need anymore help i will try to help...
NOTE: Rooting your G1 will void your warranty..