To design instruction means to make a plan and figure out how you will implement it to create an environment via a pre-selected delivery system which best creates the conditions under which your learners can learn what you have pre-determined they should know and be able to do by the end of the instruction, and which enables you to assess whether or not such learning has taken place so that you can make adjustments to the instruction both for the benefit of the current learners and for future learners.
That is a mouthful. Basically, designing instruction must begin with a plan. If you have done your job well, you will have thoroughly analyzed your learners, their needs, the knowledge and skills they bring to the table, the available technology and resources, and the tasks to be learned. With this knowledge you are able to determine the best delivery system for your instruction. You can also write learning objectives which specifically state what it is that your learners should know and be able to do at the end of the instruction. You will also be able to design the assessments you will use to determine if the learners have acquired the desired knowledge and skills.
You can then use this information to design the instruction. Whether this instruction will take place in a traditional classroom led by a teacher who stands in front and lectures and then hands out paper-based multiple choice tests, or whether this instruction consists of student-led instruction taken via an online course and assessed through an authentic, peer-reviewed project, the process for designing it is the same.
During and following the instruction, a process for feedback and evaluation is built in to the instructional process to be used to make course corrections so that the existing learners can be successful, and so that the course is better during the next iteration of the cycle. This is a critical component of the design process because if you do not plan for the evaluation component when designing the instruction, this crucial step could be left out of the process.
To design instruction you will need skills in communication, particularly listening skills, because you will need to be able to communicate with your subject matter expert and stakeholders in the project to do a thorough analysis. You will probably need to be able to work as a member of a team, because rarely do instructional designers work alone. You will need project management and organizational skills to be able to deliver materials on a schedule and keep track of materials and deadlines. You will need to have good writing skills so that you can communicate in writing your design plan to your stakeholders, and the design to whomever might be programming the instruction. You will need to have a basic understanding of the process of instruction to create the instructional objectives and design the assessments.
Ineffective instruction is instruction which was not designed; instruction which was haphazardly created without a plan and without the proper analysis prior to its creation. Although designing instruction can be time-consuming, if you take the time to do it correctly, instructional design ensures that your product has the best chance of being effective and successful.