There looks like there are three questions there..

installing aircraft is easy: Unzipo the download, read the "read-me" file.  The usual step after un-zipping is to copy and past the add-on aircraft folder (which contains the model, panel, sound and texture foldeers, along with at least two files, aircraft.cfg and thye 'air" file) into the fsx/simobject/aircraft folder.  Some add-on's also have an effect and gauge folder.  Copy the contents of those folders into the fsx/effects and fsx/gauge folders.

Adding a new texture consists of unzipping the downloaded texture file, un-zipping, and reading the radme.  Usually, you copy paste the new texture file into the specific aircraft's folder, and edit the aircraft.cfg.  The needed edits is usually included in the read-me file of the down-load

Creating your own custiom paint job involves downloading DXTBMP, or ImageTool (from the SDK), and also using an image editing  application like Paintshop, Photoshop, or equivolent (and there are some freeware apps you can use).

Create a new "texture.whatever_you_want_to_call_it" folder, and either copy the contents of the original aircraft's textures into it (and some come with blank textures or paint kits).

Using DXTBMP, open that folder, select the texture .BMP you want to repaint, send it to your image editor (which you choose when you "send to editor" the first time), When the editor opens, make your changes, then "save" (not "save as") then back in DXTBMP, "reload after edit".  If you are saving a native FSX texture, save it as a .dds, and over-write when prompted.

After that, it's just like installing a downloaded texture, except you have to create the new edit for the aircraft.cfg file, which is not all that hard.  Just copy the first [fltsim.0] sectopn, paste it under the last fltsim entry, tne change the fltsim.0 to the next available [fltsim.number]  Also change the "Title=", "texture=", "ui_variation=", and save your edits.

That's pretty much it, and that is the "easy way", even if it doesn't sound easy. (But it gets easy with practice)