As luck would have it my little book of instructions has done a disappearing act - however, I think that my memory is OK (never too sure these days!) and the info above will work. I find that shellac is a delightfully uncritical product to use.
If I were finishing fine carving or detailed features of table legs etc. I would use a fine lacquer brush of natural hair (Squirrel or Badger) and a good brand like Leonhardy. They can be obtained from The Gold Leaf factory The Gold leaf Factory International | Manufacturer of Fine Gold leaf | Art Supplies | Artists Materials and probably other places. I also see that Shines now have a (basic) web site shines.com.au and that they advertise that they sell a lot of different shellacs and brushes too. (I have no financial or other affiliation with either company, just a customer).
However, I do not believe that you need a special brush for the work that you are going to do, I have only included those details in case you, or others, need the contacts.
We have cedar doors in our house and when we had the place painted a few years ago the painters offered to "freshen up" the doors with my shellac at no extra charge. They used good quality paint brushes to shellac on the doors, so they should be OK for the stair work that you have to do.
