I suggest starting at www.evergreengardenworks.com/articles.htm for 'reading' material. You *will* ned to make a cold-frame or such to protect the roots of that tree, but the cold is not likely to harm the exposed wood, IMO. Not until it gets *really* cold
If you go the 'plant catalog' at same site, Brent lists the root-death temps for *many* species - be sure to use the index on the first page to udnerstand the symbols/numbers he uses to tell the plants' traits/limits - few places do that for their customers, or public in general. But do read up as there are many concepts to understand - forget everyting you know about 'regualr houseplants as those rules do NOT apply
You can take the pot its in and place it in a larger pot and pour bark mulch or other insulating material in there to make a 'blanket' over the pot essentially. There are many ways of protecting the roots, but it will need to be done, especially if tree is going to dormancy already. Yellowing and drying leaves aren't necessarily bad - but green crisping leaves are...big difference between the two.
You should not water by submersion - much better for a few reasons to water several times 'from above', and that is addressed in the first link above, I believe. You can also glean outstanding well-written stuff from www.bonsai4me.com as Harry is rather well versed in his approach to bonsai. He does a commendably great job in the 'species guide' section, IMO.
And yes, many sites contradict others - but take evergreengardenworks and bonsai4me as the true gospel. Tose guys kinda lead the pack in the 'how-to' on bonsai (trust me on this one, LOL!).
For now, just keep soil semi-moist at most (no swimming for the tree, OK? hehe) and if the leaves continue to yellow/drop as it gets cooler, you really need to find out more about if it *was* an outside tree as that would mean that it is going-to-sleep naturally and you *must* allow it to continue (can explain later if not understood from the reading links above, OK?). Do not carry it in and out of house each day as this messes with the timing/cycles of temps the tree is genetically programmed to respond to normally. Practically all deciduous trees require dormancy, and those folks that refuse to honor this tree-demand always come back and post "What happened to my tree?" - and its a no-brainer as to why it died
I am sure you will have more qauestions that we will be happy to clarify for you - but please statrt at those two sites for a much more 'balanced' background of 'bonsai basics'..
Again, welcome to the site. We'd love to see some pics if possible (hint, hint) but not mandatory by any means..
And it is OK to remove 'dead' yellow leaves usually, but better to let Nature take its course as the leaf-stem base upon branch generally seals itself naturally eventually, and early removal may possibly be semi-bad (but not fatal by any measure, IMO)... As tree loses more and more leaves, the need for moist soil drops proportionally - so careful to not overwater
Only water when soil is dry a bit under surface of soil. There is no way to water by a schedule - let tree/pot indicate when it is time.