Early Game
Doomy is great at jungling.  Grab Crime Bible and Marauder Shield and start on the elites if you can get a leash or any of the other camps. Keep in mind if you run into their jungler or their jungler has aid you don't want to engage move on to another creep unless you see that they are weak and then by all means give it a go. DD isn't much of a threat till he has all 3 of his abilities.  

When jungling be sure to time your stomps between AA's to maximize damage and clear times. You will be taking W first for the maximum jungle clear, that dot is awesome. 

Farm up till you can get both the Crime Bible and the Marrauder Shield to T2. Then go back in see if you can land some ganks now that you have your Charge, Stomp, and Debris field. As fast as you can farm up 2k  to grab your collectors. Ruby of Life and Power Beads these will give you HP regen/HP/CDR and Power/Attk Armor, making you pretty Tanky and disruptive like a good jungle should be.

Mid Game
You  go back to the jungle make sure your not falling behind in experience. Farm up the spear of Destiny and max it out as soon as possible. This is your damage right here, with out it ur just a stompy fool who does nothing to no one. Once you have it maxed your HP become your weapon and now you don't just make a lot of noise when you stomp but if you hit your combo against a wall you are breaking necks. Keep up pressure on the enemy team. Make sure you challenge map objectives like the elites and the relays. You are tanky enough by yourself to fight 2v1 or 3v1 till the cavalry comes to help you with relay caps you are contesting. 

Late Game
You should be pretty farmed by now. You are pushing lanes making sure relay caps are going smoothly. Position your self to stomp on squishies who are out of position. Make sure that position ends with you slamming them against a wall or other unforgiving surface.  That charge of your is excellent for interrupting ultimates so save it if you didnt initiate. Consider building CDR so that your abilities are up when you need them. Batman's Utility belt is great for this. CDR/Health/Attk Spd are great things. Build up your items past T2 if you find yourself fed. The end is nearing for the enemy team if everything is going well you will be stomping their power core in shortly.

Team Fighting
When team fighting Doomy's pretty simple - identify the squishiest target you can find or target you can get ur full combo on and hit em with E, then W, Q. If you weren't already popping Kryptonian Rage then do it now. Watch who ever you are stomping on not get back up. Pretty simple. Prioritize damage dealers and ppl that arent stacking Attack Armor and crush away. 

The Optional Items

Lobo's Chain - Excellent Item more HP and More Damage with T4 perma slow. Only problem is its expensive and if you are hitting your combo right they aren't getting away. It's a good choice if you're ahead on cash and feel that you are a little soft or your prey is getting away too often. 

Batman's Utility Belt - Best Item evarr! The belt has HP/CDR/Attack Speed/and Move Speed as well as a Purge/Cleanse there is no reason not to grab this as you last item. 

Psi-Scimitar - this item is great for putting more umph into your punches. Since Q can proc the effect you can double dip your damage on this item. DD also has great ratios when he adds power as well so its something to consider if you feel you arent killing fast enough.