You can find a guide
here (with pictures!) that steps you through the procedure to remove the logic board's heatsink. That needs to be done in order to gain access to the CPU and GPU that will be having their paste re-applied.
Step 28 has a link to the guide on how to re-apply thermal paste.
My personal tips for anyone doing this for the first time: It's not a race. Don't rush it. Take your time. Use the proper tools (don't try saving a few bucks by taking shortcuts with tool substitutions.)
There was another thread on here recently where somebody had taken the bottom plate off their system and observed airflow patterns on the inside of it (dust patterns that clearly showed the airflow pattern). Those patterns suggested that placing the cuts near the sides of the panel would have the greatest effect. I'm not so sure that putting that spiral cut will provide a better benefit. It will probably make a diff, but I think the best effect will be from the side cuts. I would just do them on the side and test the results before going any further.
But, before doing any cutting, do the thermal paste. You may just find that the results from that alone are good enough for you, and that you won't want to mod the bottom plate after all.
Also, for me this goes without saying, but with increased air intake comes increased dust intake. So
more frequent cleanings (most people never ever clean the inside anyway) will be necessary if you end up doing the bottom plate mod ... a can of air sprayed across the system with bottom plate off should be sufficient from time to time. No need for anything major like an ultrasonic bath.