Making a sourdough levain is easy, you just have to get accustomed to the timing. Once you mix up your final bread dough, it will have to rise, be shaped, then rise again before going in the oven and cooling before slicing. The whole process will take at least 6 hours, but closer to 8 if your starter is new. Therefore, you will have to mix up the levain 6 to 10 hours prior. Prolonging rise times is not a bad thing, it can make better bread, so it is always better to plan on needing more time.
If you are having guests over for dinner and you would like fresh bread to slice at 6 pm, you should mix up the levain at night before you go to bed. When you wake up in the morning, mix up the dough and proceed with letting it rise. This is normally how I bake bread because it leaves me ample time during the day to watch the dough. You are also giving the levain a long time to rise, which is preferred to trying to rush the process along