Measure your loft hatch.
Measure, accurately, the distance between the joists, and their thickness.
Get some 18mm ply at a place that will cut it to size. You want at least two pieces that are as long as you can lift through the hatch, and about six inches wider than two joists.
Screw wooden battens to the underside of the boards, such that when you lay a board two joists, the battens fit loosely round the joists so the board cannot slide sideways off them, and cannot tip over.
Drill some one-inch holes in the boards which you will use later to lift them with your fingers.
Using a huge pair of scissors, cut a slit in the loft insulation on the path you need to take
And fold the insulation back each side to expose the joists.
Place your first board on them
Stand on the first board and place the second board further along your route.
If you made enough boards, you can pave your entire route. If not you will have to lift and reposition them.
You can also make a floor with lengths of decking board, which is quite cheap, light and strong. You must not have unsupported edges projecting beyond the joists. Any boards that are not screwed down or restrained by battens are liable to slip off the joists and give way.
If you can afford it, get larger boards that will span three boards and make a wider, safer path. Ply comes in sheets about 4 feet wide.
The more joists the boards are screwed to, the better they spread your weight.
Never attempt to stand on insulation when you can't see what's under it. Hence the cut and fold. You can put it back when you're finished.
For a permanent job, you can screw the boards to the joists, taking care to avoid any cables or other obstructions.
Chipboard might be cheaper, but not so good.
