Order ahead is a second door into a counter you already run. Most of the work is deciding where the order lands and how it gets handed over.
The first thing to settle is that order ahead is not a separate operation. A guest picks items from a code or a link, pays, and the order prints at the station that makes it - the same station, the same printer, the same kind of ticket the register produces when somebody orders in person. Treat it as a second system and you get two menus, two sets of totals and staff who trust neither. Treat it as another door into the counter you already run and it is far easier to staff.
That framing answers most of the setup questions below. The register stays the main way orders come in. Order ahead sits on top of it, and you should be able to switch it off at one counter on a quiet night without anything else behaving differently.
Then decide which items belong on the ahead menu at all. It need not be the whole menu, and at most venues it should not be. Items that hold, travel in a bag and need no question asked work well; anything needing a choice at the counter is better left off.
An ahead order goes wrong at the handover far more often than it goes wrong in the software. The guest needs to know where to collect, how they will know it is ready, and what to show when they arrive. Put the counter name on the confirmation in the same words as the sign, say plainly that collection is at that counter, and give them something to present - an order number large enough to read across a counter, or the confirmation on their phone. Staff should be able to match that against the ticket without asking for a name twice.
Say where and how, not when. You do not control what the kitchen is in the middle of, what the stage is doing or how long somebody takes to walk over, so a time on the confirmation is a promise made on somebody else's behalf.
Before guests see it, place real orders through it yourself. Stand where a guest would stand, on the venue's own signal rather than the office wifi, and order the way they would. Watch the ticket appear at the station, make the thing, shelf it, then collect it with the confirmation in hand and nothing else. Do one awkward run too: two stations on one order, an item with a modifier, one placed while the counter is busy. That half hour finds the counter name that does not match the sign and the station nobody pointed at a printer.
Last, decide what happens when it is off. Signal drops, a counter closes early, an item runs out. The answer should always be that the counter carries on taking orders at the register as though order ahead never existed - that is the part which must never depend on it. Pull a sold-out item from the ahead menu the moment it goes, and make sure one person on every shift knows how to do it without being talked through it.