Start-up costs – guidelines?

What are the guidelines for entering start-up costs? I read the article in the learning center about this, but its explanation is fairly general. Do these costs have to be incurred within a certain date before the start-up date? Does this only apply to larger investments (e.g. laptop) or can more everyday costs (e.g. public transport) also be claimed, provided they have a clear link to the professional activity?

I am not subject to VAT, so for clarity’s sake, I am only talking about the tax deduction of those costs.