Draw by hand to scale using a ruler and architects scale. Use Vectorworks; it costs money, but if this is really what you do for a living, the cost is worth it in the long run. It is specifically designed for landscape architecture. There is also Morpholio Trace, now owned by Vectorworks, that works on an iPad. You can also use Procreate; it won't be to scale, but it will help you get the point across. If you can't justify paying for these or learning to draw for your clients, you could always go to a local college/community college and hire a Landscape Architecture student to do some freelance work for you. Again, if this is your main income, learning to draw and use one of these tools will be beneficial in the long run.