So here is my quick run down how I made mine without any extra extra using what Windows comes with (and Aida64 of course) and what others have offered here.
Find a picture/background you want to use one your panel. Use windows photo to resize it to the size you need for your panel.
Use Adia64 to build your panel. It is built by layers. Bottom most being your back ground with everything else atop it as the next layers.
Adia64 has some stock gauges/graphics etc you can use. But let say you see a set of gauges you want to use on your that someone has shared and already created. Download the file they shared, import that panel to adia64. The program will save all the images contained in that panel to your Adia folder (where you saved Aida on your PC) and can now use them on yours. You can also resize each gauge using the windows photo editor.
The layering in Aida is on an X/Y axis (updown/leftright). You use the Aida editor to move your items in to the position you want.
Aida will automatically use the sensor for what you wish to monitor by you selecting the item you want to monitor. You than need to assign EACH picture of the gauge to that monitor (This sounds worse then it is). In Aida when you pick the monitor sensor, you can select each picture of the gauge (that you saved and resided earlier). Remember it is layers, so when the temp or whatnot goes up, it looks for the next picture in the gauge file you selected to give the visual of a gauge scaling up (in reality it is the next picture layer showing).
I would say dive into it, give it a go. Once you start messing with it, it makes more sense. It looks REALLY hard on the surface. But these are simple to create. Now some of the more in depth ones you see from @Exhumed and others, they (am assuming) using more software such as photoshop or Coral draw and other editing programs with much more capabilities (but they also pay for these programs and am guessing they are graphic designers or something along that realm).