First, it might be helpful to checkout the forum dedicated to this subject:
It's probably ideal to upgrade to 3.7 SP4 before attempting the leap to Sitefinity 4.2.
Regarding the actual migration, there are some instructions embedded in the BETA Migration Tool inside Sitefinity 4.2. Give those instructions a look and please post questions.
The entire 3.x -> 4.x migration story could be described as "evolving". We're interested in working with early adopters, however expect to encounter bumps in the road. If you're looking for a seamless experience, then I recommend waiting for the official release of the migration tool. However, if you're willing to help us test, then we welcome your involvement and questions.
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