1. To mark a short level line, you can use a level to extend the line from a measured point. Longer lines are best marked with a chalk line. Drive a brad at one measured point and then attach the chalk line; reel out the line to a second measured point, stretch the line tight, and snap the line on the wall. Then check the line with a level.
2. Once you’ve marked a line, use a 4-foot level to check it. Lightly mark a level line in any areas of concern and measure the difference between it and the original line. If a chalk line is “out” by only 1/4 inch or so in 6 feet, you can probably go with it. Usually the eye will accept mouldings that follow the floor line within these limits, especially baseboards. If the difference is more, use the level to re-mark the line, setting the line level or perhaps splitting the difference.
3. Remember that in moulding work the goal is to produce a pleasing result,not a mathematically perfect one. It’s a matter of eye. Your care and attention at this stage will be well rewarded down the line.