I had no experience with photoshop at all so i feel your pain. I made a bunch of NES covers my own personal style so here's what I did (in reduced, quick steps)
1) download the templates provided in this forum
2) download existing covers already posted, and cut away/erase the logo for the spine and the front of the game (can do the same for the developer logo - magic eraser is a blessing!)
3) scour the internet for good custom artwork (deviantart, etc...) for each game.
4) grab a screen capture off of an emulated game, clean up that image in photoshop/imageviewer
5) insert your respective images (logo, artwork, character sprite, developer logo) onto the template
it helps to practice "level control" when assembling your photoshop design. that way, each individual image/logo/etc is on it's on layer and can be moved freely. also, it's nice so that when you go from castlevania 1 -> 2 -> 3 you can just "save as" your previous game cover and the developer name/background color are already done for you.
At first, it took me a while to complete a design; but at the end I got pretty good/fast (because I only use a few photoshop buttons/options) at it. Of course, i wasn't going for the retail look and was making my own. hope that helps somewhat.