Skirting boards on uneven floor

Hello Jk, always follow the level of the existing floor and sit it flat against the floor, you will have to accommodate for this un-level floor when you cut your internal scribes and external mitres don't try to get the skirting level because over a long length It will sit flat one end a be off the floor the other and your eyes will draw to the gap underneath straight away, the only time you will have to scribe the bottom of the skirting to get it flat against the floor is when there might be a slight hump or imperfection in the floor (for example a tiled floor in which the tiles are not perfectly flat against one another and pertrude up 2-3mm) or the skirting has slight warp, this can be overcome by scribing the bottom part of the skirting to get over these issues with the use of a hand plane or jigsaw. I hope this makes sense.