You can copy someone's trade from anywhere in the app you see a trade ticket.
The most common places to see a trade ticket is on your Activity Feed and within any Groups you're a member of.
To see trades on your feed, you must be following someone.
To copy a trade, just tap the Copy button underneath the trade ticket.
You'll then be taken to the trade execution page and see the original trade above the entry fields.
Once you're happy with the trade details, just tap "Trade".
That's it, you're done!