Open the Materials Browser / Editor... Create a new material [click the '+' button]. Give it a logical name. This is to use one of your specific texture files. Tick the 'use texture' box and navigate to the texture-file [jpg, png etc]. Add that image-file as the material's 'texture'. Adjust the size of that texture as you desire [break the 'chain' symbol if you don't want 'proportional' resizing - aka 'distortion']. Adjust the material's 'color' [as used when no textures are being displayed]. Adjust its 'opacity' [aka transparency!]. If you want a texture to 'tile' nicely - that is, to repeat across a larger face without noticeable patterns showing up where one part ends and the next one starts - you must either get a 'tile-able' image in the first place, or make it into one - there are several tutorials around on how to do this with Photoshop/Gimp etc... If you have a material in the model that you would like to 'export' individually, select its icon in the model-pane of the Materials Browser, right-click context-menu and 'Save As' to put it [as a SKM file] into a folder within the Materials folder [either an existing folder or one you create] - then it'll become available in all other models - just like the standard materials shipped with Sketchup. When you have a collection of materials in the model that you would like to 'export' en mass then click the right-facing arrow on the side of the Browser and the pop-out menu includes 'Save collection as...' - this saves all materials in the model into a specified folder [you can make a new folder as you do this if desired]... Again the SKM materials in that folder become available to all models thereafter...