I also have a 1993 LS 400. I changed my filters about two months ago.
Directly in front of the passenger front seat, you will notice a lower panel. This needs to be separated from the upper part of the dash. As I remember, there are three clips at the front part of the panel and two towards the rear. Use a putty knife and very gently pry the panel out. Be careful, though, because the clips that hold it are very fragile. (Two of mine were already broken.) There also is a light in the back part of the panel, so be careful removing it.
After removing the panel, you will see a plastic housing with wingnuts at the bottom. Remove these wingnuts and the bottom of the housing will come off. The filters are located in this housing. One of them will probably drop out when you remove the bottom. You may have to pull on the other one to remove it. The filters are designed to bend when you are removing and installing them.
The new filters come with an instruction sheet. I ordered mine from Irontoad.com. I believe they were about $35.00.