Cut lengths of contact paper that equal the height of the trees you want. From the top of the baseboard to the bottom of the molding, my walls measure 88 inches. So I cut four or five pieces that were 24" wide (the width of the contact paper) and 88" long. Since each piece would yield 3-5 tree trunks, I'd have 3-6 trees per wall.
Create a bark pattern to the contact paper. Cut a sponge stamp in the shape of an eye with a hole in the middle. You can stamp the knots in the tree's bark and use the thin edge of the sponge to create lines radiating out from those. My paint was a mixture of yellow with a smidgen of black, so it's not a true black. I didn't want a dark black, as in the real world trees have all sorts of living things that give the color a little warmth.
Cut trunks. This is the sheet before I cut it into 3-5 long trunks. To cut the trunk-strips, I used the grid side of the contact paper and cut roughly along one line, allowing the scissors to wander off the line a little bit because trees don't make straight edges in real life! I also added some wiggle to the edges of the original contact sheet too, since a machine-made edge would look weird next to my wiggly edge. Advanced mural makers may want to add a very gradual slant to the trunks, so that the trunk is thinner on top than on bottom, and alternating up and down to conserve contact paper.
Create leaves. To paint the leaves, I started with an undercoat of yellow, then regions of sponge-painting: a little red, some green and lots of orange over the yellow. It's a little like an Eric Carle effect. I also added some metallic gold sponge-brushing to the paint. When this dried, I cut out heart-shaped leaves. I was so happy with how this looked before cutting I wished I'd done it on canvas. Another time.
Cut leaf shapes out of the color sheet you painted (I made vague heart-like shapes because I love my boys and I love that aspen trees have heart-shaped leaves. My trees are somewhere between aspen and birch.) This part can be tedious as it's tough to peel off the backing on the leaves. I put up about 30 of them and then decided that we should wait to make this a ritual for guests who came to meet the kids--that each visitor adds a leaf to the forest.