While you could purchase a basic K'nex set at almost any toy store, K'nex also sells special educational K'nex sets that are designed for classroom use. The math and geometry sets include pieces needed for teaching angles, while early childhood sets teach hand-eye coordination, physical science kits include bridge models, life science models include DNA models, and so on. Think about the curriculum you are using this school year and base your K'nex purchase on your plans to ensure a good fit.
Once you have purchased your K'nex set, you can move on to deciding on specific lesson plans to use with your homeschoolers. If you purchased an educational set, some curriculum will be included for your use. If you did not purchase an educational set with included lesson plans, consider using pieces to demonstrate the various angles from Geometry, letting your homeschooler try her hand at it. Find diagrams of DNA and atoms and ask your child to use the K'nex to make a model of those things and explain them to you. You might even ask younger children to use K'nex to create the alphabet and then take a photo to commemorate the experience. Make sure that your K'nex lesson plans intersect well with what you have already planned in your curriculum so that your children will have continuity in their educational experience.
In preparation for doing the K'nex lesson plans you have chosen, be sure to prepare a large clean working space. K'nex pieces are easily lost in clutter, so a clear table top is the perfect working location for most projects. Make sure that younger children who might choke on smaller parts are away from the project area, and ensure that pets are in another area of the house so as not to disturb your homeschooler's progress. Engage in the learning project with your homeschooler and consider inviting a few other homeschoolers over so that your child can experience teamwork on larger lesson plans, such as building roller coasters or bridges.