If you are going to insulate and attic with fiberglass insulation you need to decide if you will apply it to the roof rafters or the floor joists and measure the height and width.
Measure the rafters
In either case you need to know what the depth and width is between them so that you can purchase the correct width of insulation. You will find that newer homes are mostly 16 inch and 24 inch center spacing and have a depth of 3.5 or 6 inches.
Many older homes do not have a standard size so the depth and widths will affect how you will install the insulation.
If you are putting the insulation in the roof rafters you will have to install spacers before installing the insulation, to allow air to circulate between the roof and the insulation. This is necessary to eliminate humidity build up in the insulations from condensation.
Put the spacer between the rafters and staple them into place so that air circulates from top to bottom.
Install the insulation
Then measure the length of the rafter with a tape measure. Unroll the insulation measure the length you need put a piece of wood on top of the insulation, press down and compress it and cut along the edge with a sharp utility knife.
Place the insulation between the rafter and unfold the paper vapor barrier edge so that it rests on top of the wood and staple it. Place the staples about 8 to 12 inches apart along the entire length of the insulation.
Older home installation
If you have an older home and the insulation does not exactly fit between the rafters it may require you to cut an extra piece to fill in the space between the rafters. To hold it in place you will have to use straight thin metal spacers placing them on top of the insulation and flex it slightly so that it fits between the rafters to hold it in place.
It is also a good idea to use a mask to protect your lungs and protective outer clothing so that little pieces of glass gets into your skin or else you will be scratching like a dog for a few days reminding you that to never do this type of work again.