I like to use a table saw for cutting the bigger sheet goods it gives a nice clean edge to the material, it is square, usually easier to work with than say a skill saw. Lay your piece of hardboard out & figure which is the best way to cut it where you waste the least amount of material. I always figure the best way to save on material & most of the time I can get multiple uses out of one sheet, sometimes I only buy half sheets & still can make multiple structures out of it.
Now use your pattern as your size guide to line up your fence on your table saw. Using the patterns saves time from having to re-measure everything & less chance of making a mistake. Place your pattern for your side walls between the saw blade & the fence & this will give you your height for your side walls, next do the same for the roof panels & also the end walls (for the width).
If you figured properly you can cut a strip & this will give you the material needed for both side walls, both roof panels, & both end walls. Now use your table saw miter gauge to cut the lengths of the side walls & the roof panels. Then use your miter gauge to cut the peaks for your ends walls depending on what pitch you decided to use for your roof.
I made the roof with a 6/12 pitch which means for every 12”s it slopes 6”, on the miter gauge I used 62˚. It is a little awkward using the miter gauge to get these cuts, if you turn your material upside down or place the miter gauge on the opposite side you will get it. You just need to flip your pattern back & forth or turn it around to get the right angle for the cut. When making these cuts make both the same cut for the front wall as the back wall before flipping it over to cut the other angle, this keeps you from having to change your miter gauge for each cut.