How to correctly add flooring to insulated attic
The alternative to Eric Simpson's simple solution is a product like the following. (This particular one is from Amazon, but searching for "loft flooring legs" will find many like it.)


Plywood/chipboard is often sold in 2400x600mm (8'x2') sheets precisely for fitting through a hatch to floor a loft.

Also, beware!  Roof trusses are usually designed down to a price, which means using the thinnest pieces of wood which can take the load of the roof.  They are usually not designed to carry substantial loads like a standard floor and a massive pile of junk on top of it.  If you want to put in a pathway and store empty suitcases and spare duvets/sleeping bags that should be fine; a complete floor + piles of old books - not so much.
(Thanks to isherwood for the comment.)