I'm not sure you can earn DEC outside of the game.

A simple strategy for the game is ...

Play your shield in the front postion (This is generally the card with the most life and shield, maybe use the heal ability.)

Second position play a card with the reach ability. This card will attack the opponents card in the front position.

Third position use cards with magic or opportunity abilities. ( They can generally attack from any position.)

Fourth Fifth & Sixth positions are good for cards with the sneak ability and the archer or arrow cards. (These will attack opponents cards in the rear of the battle and also snipe magic cards depending on abilities)

This is just basic strategy, learn as you go. You'll come up with your own strategies as you learn the cards, their abilites, and how to position them. Here's the general game faq.

https://splinterlands.com/?p=faq

Splinterlands Grind on Twitter is always posting good strategies too.

https://twitter.com/SplinterlandsG

If anyone has any Splinterlands tutorial pages, feel free to link in the comments.