If you are in good shape and have done plenty of bushwalking/scrambling then you should be fine to complete the walk in a day, just bear in mind that you will need to keep on the move to give yourself enough time as you must ascend (and then descend) 1000 m!
In terms of navigation, the first couple of hours are very easy - you follow a dirt road to the base of the mountain, which eventually becomes a wide path at the start of the south ridge (there's even a sign at this point). As you start the ridge proper the path becomes less obvious, but there are orange markers nailed to the rocks at certain places to keep you on the right route. The higher up you get, the more scrambling involved. There is one section with a steep, wide rock slab that I have read is difficult in the wet, but you should have no troubles with in the dry as there are indents in the rock where you can get good holds.
At the saddle there is an obvious campsite where the ground is cleared - if you have started early enough you should be fine to make the east summit from here. I would allow about 2 hours return to this point, although you may need more time if you want to sit up on top for a bit. There are several faint tracks to the east peak from the saddle, but as long as you head in that general direction you should make it fine. Be aware that whiteouts are quite common here, and this is where having a compass and topo map becomes important. Also worthwhile packing a jacket etc as the weather can change quickly near the top, even if the rest of the south east is clear. When returning to the saddle simply aim for the west peak and you will end up back in the rainforest near the creek. Be careful not to head too far north at this point. When you descend from the saddle back down the South ridge, remember you need to ascend for about half a km first, don't take a downhill path or you will not find the correct track.
As with climbing any rocky mountain, I would give it a miss if the weather looks iffy. Better to choose a fine day, get great views and avoid slipping on wet rock.

Mt Maroon is definitely a worthwhile walk in its own right, if you are not confident for Barney right off I would give it a shot first - you certainly won't be disappointed with the views from the summit.