Quite a few things can be going wrong. First, if you are using starch or sizing you need to press and starch your yardage BEFORE any cutting. This is a big reason I prewash, to remove the factory sizing and to help release the factory fold for the bolt which is usually out of square which results in wonky strips with dog legs and waves.
Then you need to make sure you are folding your fabric correctly. Do a google search and use "how to fold fabric to cut strips" as your search criteria. Several YouTube videos will come up as well as blogs with pictures to help you.
Then google "how to rotary cut fabric" for youtube videos and blogs that offer tips for how to hold your ruler and rotary cutter to avoid slippage or moving your ruler a bit as you cut along the edge. Some people prefer to put little non slip rubber dots on their rulers or dots of sand paper. I prefer nothing on my ruler and will splay my fingers out for increased weight distribution. I also walk my fingers along the length of the ruler as I cut.
You have to find what works best for you when it comes to cutting but the folding part is crucial when you start. A crooked fold results in crooked cuts.
If you are getting waves from sewing your strips together, it helps to reverse the direction with every other strip.