There is no easy answer for that.
You need a plan. Get a list of the companies standard layers and properties. If they don't have one, make one of your own. That's the easy part. Depending on how the drawing was created, you may have to go into each block and redefine it so that it meets those standards. Layer merge might help also but it depends. There are other things to consider such as linetypes, blocks within blocks. It really comes down to what you have and being clear on what is needed and developing a strategy for getting there.