Hi, I'm not an expert either, however I have been "fostering" a friend's hibiscus tree for about a year now. When I got it, it was in bad shape with withered leaves and yes, they stared turning yellow. I plucked off the yellow, dying leaves so that the tree could expend it's energy on new, healthy growth. It was actually bare for like a month. After researching, I decided to make a large humidity tray to put underneath it (I used an old christmas tree tray and filled it with rocks then added water). Be sure the rocks are placed so the pot can still drain. Then I added a little bit of new soil on top of the soil in the pot so that it could get more nutrients when being watered. Also, I have a spray bottle and generously spray all the leaves top and bottom everyday. They seem to like a lot of water and LIGHT. I moved it to the sunniest room we have. Remember, they are tropical plants really so try and channel Hawaii conditions: Warm, Sunny and Wet. My husband and I called this "Hibiscus Hospice" and this spring it has grown new green leaves and had 43 buds on it at one point! It is still not completely full of leaves but after thinking it was a gonner I am so happy at where it is now. I hope this helps somewhat, good luck. Here is a before and some after shots.
