Best to work the timeline backwards. These are absolutely best case time frames.
Start early Jan 19.
Give notice to starters early Dec 19.
Have completed assessments early Nov 19 in order to review and pick your best group to kick off.
If there are (say) 1000 applicants they will interview about 500 or so. That will take at least 3 months so we are back at early August.
Those 500will he picked off the back of online testing completed in early to mid July which means applications should open about June.
Of course you can shorten that timeline a bit if you’re only going to take (say) 8-10 in the first month and then keep working the process as a selection ‘just in time’.
There’s also significant risk in recruiting for a training scheme where you don’t know where it’s to be held or who is going to run it.