Contact paper provides the perfect medium for stick-on stencils when you want to create your own stencil designs -- it's fairly inexpensive, easy to work with and is sticky enough to ensure no paint seeps beneath the stencil while you work. A craft knife gives you the most control when cutting out curves and details once you're ready to cut out your stencil.
Sketch out a design for your stencil on paper. Once you're happy with the design and its size, go over all the outlines with a permanent marker. A computer printout of an image to stencil is another option if you don't want to draw freehand.
Place the paper design on a sheet of scrap cardboard on your work surface. Cut out the shapes or letters, including tiny detailed areas, using scissors for large cuts and the craft knife for details. If the design requires particular layout or spacing, such as a word or phrase, cut the shapes out of the paper without slicing through the paper "framework," so the paper itself looks like a stencil when finished. For simple singular designs, such as the silhouette of a bird in flight, feel free to cut through the paper to cut out your shapes.
Smooth the contact paper out on a flat surface, face up. Place the paper cutout(s) on top of the contact paper.
Trace the paper shapes or cutouts onto the contact paper with a permanent marker. Hold the paper steady while working, or tape the edges down with painter's tape if you find it difficult to keep the paper from slipping as you trace.
Slide the contact paper onto a sheet of scrap corrugated cardboard. Cut out all of the shapes using the tip of a craft knife; the knife tip allows you to cut with great control without requiring slices through the edges of the contact paper. Cut just through the contact paper, not the backing, whenever possible. When finished, carefully peel the paper away and smooth it onto your project surface, ensuring the paper sticks well without bubbles or gaps.
Turn a regular non-sticking stencil into a stick-on version by placing the stencil atop the contact paper, tracing the outside edges and all cutout areas with a marker, then cutting the shapes out with scissors and a craft knife.
If you're confident with your design idea, sketch it directly onto the front of the contact paper with a marker instead of making a paper template first.
The tip of a craft knife is very sharp. Place a piece of cardboard or sturdy shipping foam beneath the contact paper, setting both atop a firm surface when you cut. Do not place the contact paper in your lap or directly on any furniture when cutting.