To find the minigame, you have to access it from the screen when the game first loads up! I couldn't find it for a while myself.
My experience was that this is slightly difficult; there's several things to keep in mind that should help you get this first try!
1. If you Do Not move your tank, the aiming reticle gets smaller.
2. If you Do Not move your turret, your aiming reticle also gets smaller!
3. Combine the above two whenever possible to shoot enemy tanks that are still off the screen. They won't shoot you until they are well on the screen--spawn kill them!
4. When you start getting into level 4+, you need to kill the tanks really fast, try to take them out almost as soon as they spawn. If you don't, the tanks will build up in number and can get you trapped!
5. The perks that drop from tanks are VERY useful depending on what drops. They will disappear after about 10-15 seconds though, so snag them quickly if you can. If you get a camouflage, hang onto it for level 10 though, it will be VERY useful when the German Maus shows up.
6. Each wave increases the number of enemies, until wave 10, where you'll get one Maus tank which takes a LOT of hits to kill, and unlimited other tanks to distract.
7. I found running a circular pattern would keep me from getting hit most of the time. You can even stop and go to dodge shots.
I hope this helps!! Please let me know if there's anyway to improve.