Does anyone have a good guide for importing historical trial balances and opening balances?

HTBs are just journal entries; they can be posted as YTD for each period or delta aka activity for the period. with YTD approach you would reverse each journal except for the last period where you only reverse Balance Sheet plugging retained earnings to balance that TB.

Thank you @Karina! Do you recommend the YTD/Reversal approach or importing delta activities?

it depends what you have in terms of setup

I'd recommend the delta activities. Here is my website, which has some helpful guides on how to do this: http://www.intheblksoftware.com/implementation_resources

Thank you @Paul Giese! Will check it out

Happy to answer questions - my firm specializes in this process