You will be invited to complete an online form with your various details, some time after you order the car (and get the online acknowledgement). That form includes a question about whether you have a trade in (and seeks details of it). That is when you notify them. For me, they came back with a price, without asking for a condition description or seeing the car. They reserve the right to amend the price offer, but in my case they did not. My "net to pay" sum included the trade-in money. First time they saw my car was when I drove it to Tesla Edinburgh to collect my 3, where I simply handed them the keys. Heard no more about it.
Incidentally, I felt my trade-in offer was fair. Others in the forum did not think that.
Best of luck.