The good thing about a formal tender response (ITT, PQQ etc) is that you know absolutely when the job has to be done by. So when putting the project plan together it is very straightforward to start at the end and work back from there.
However, the temptation when starting a complex task of this nature is to start by getting on with the big elements, and deal with the leftover bits of detail later. Caution – it is these bits which may be the most time consuming, or which, even more importantly, may require a lead time. If you leave them to the last minute, and the lead time exceeds the time left, woe betide!
My tip is, plan all the tasks from the end back, and work out which ones need starting first. Once started you will then be able to get on with the big elements with the comfort of knowing that everything is under control.