In the following example I will be putting the texture files, your JPG's that will actually show up on your item, in a Poser Runtime folder - this may seem contradictory as we are creating a Daz material set, but it does no harm. Studio can find them there, and it may be useful for both you should you wish to later create Poser MATs, and for any of your users that want to make Poser MATs with them as well.
The folder structure you need to set up should look something like the image to the left, (I've made it in my root 'c:' drive, but you can make yours anywhere) and the only real difference between Studio 3 and 4 that concerns us is the cosmetic name change of 'content' to 'My Library', which was a transition thing from Studio 2. As Studio 3 also contains a 'My Library' folder which it uses, we'll use that as a base. I've made a small empty folder setup like the one in the picture which you can download in a zip\public\zips\ if you want, so you can use it again and again. (Although I suggest changing 'Yourname' and 'Yourproduct'!).
In this tutorial, I will be discussing how to create your own textures (materials) for items inside Daz Studio, saving them, and making them ready to share with other people! Please note you will need a familiarity with Windows explorer to create new folders, rename them, and copy and paste files.

For me, the folder structure is the
most vital part. If your setup isn't correct, the materials you save may not work or even show up!  There are some minor difference we'll find between Studio 4 and Studio 3, but I'll cover that.
Personally I like to create an entire blank folder structure, away from your main content, for a couple of reasons. First it means you can clearly see all the files in one place, and will not have to hunt around later copying and moving folders with all the rest of your content confusing you, and secondly it makes it very easy to test when you're all ready to go.

A Beginners Guide to Making Freebie Materials in Daz Studio
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Newer Studio 4 structure
Next there are two options once you have your 'final' folder setup all ready. You can choose to go ahead and start applying your new materials to your prop/figure, using existing folders in your library and then carefully moving out all the work you've done into the folders you just created in step 1, or my preferred option (to save hunting around for files amongst all your existing ones) is to add your blank structure as a new content library inside Daz and save everything there, thus making sure all your files are conveniently gathered together, ready to save and ZIP when you're finished.
Studio 3 content manager, click to enlarge
Studio 4 content manager, click to enlarge
It's really easy to add your new blank folder setup to your library, and above there are examples of what you need to do in both Studio 3 and four. I'll deal with 3 first. In the top menu, you need to go to Edit|Preferences, or press F2, then click on the 'Directories' tab at the top. Make sure the 'Content Type' drop-down is set to 'Daz Studio Formats (Native)', then click the 'Add' button at the bottom. Find where you created your blank folder, select the top level 'My Library' and click 'Ok', then 'Accept' and you're done!
In Studio 4, you also go to Edit|Preferences, or again can hit F2. Click the 'Content Library' tab, then the 'Content Directory Manager' button near the bottom. Highlight 'Daz Studio Formats' in the main pane, then the 'Add' button, and as before find your blank setup, highlight 'My Library', click 'Select Folder', then Accept and you're ready. On the next page we'll see what to do with our new setup!