Well a few things come into play here. One is that the while the Walther airgun is nice, it's really a novelty item, not a real gun. The accuracy of the airgun itself, combined with a somewhat rough trigger, could have a big affect on groups. I thought about getting one so I could shoot in my garage, but the lousy trigger kept me from getting one.
I also feel that recoil on a real gun will change things greatly. Now if you feel that the accuracy is good enough, and can live with a rough trigger, you need to work on techinique.
I'd suggest starting with the basics from these articles:
http://www.americanshooter.com/Features/RL1/rl1.html
http://www.americanshooter.com/Features/RL3/rl3.html
Keep in mind most people are going too fast when they try to do, doubles, triples, etc. The key is get a rythmn, no matter how slow, and once you get that rythmn, then start speeding up.
A saying one of my FIs had was, "At any range, if you're shooting two inch groups you're probably going way too slow, but if you can't keep it all within a eight inch circle you're definitely going way too fast." He was also fond of, "Slow down and hit the ****ing target!" As many students were rushing, and missing the eight inch plates.
