Although not particularly difficult, cutting a slot in wood is one of the less intuitive projects you can take on as an amateur woodworker. Professionals use specialised--and often expensive, tools for the job. However, you can cut slots effectively using standard home workshop tools. Buy extra wood the first few times you try this, since you can expect to make a few mistakes.
Mark the borders of the slot you want to cut. If you have a template, position it and trace the template with your pencil. Without a template, use your straight edge and measuring tape to make the mark.
Drill a hole through your lumber in one corner of your slot. If possible, use a drill bit that matches the width of your slot. For wide slots, you may want to start with a narrower bit and work your way up to avoid splitting the wood.
Drill a second hole in the opposite corner of your slot. Use the same drill bits you did for the first hole.
Attach a narrow blade to your jigsaw, one that will fit through the holes you drilled. If none of your blades will fit, drill a second hole that overlaps one of your original holes like a figure eight.
Slide your jigsaw blade into one hole and cut your wood along the line you marked. Follow the line all the way to the hole in the opposite corner.
Repeat Step 5 to drill along the remaining line.
These instructions are for cutting slots all the way through wood. For slots that don't pass through, use a Dremel or similar tool to carve the slot.