I just use some kitchen shears and a spoon instead of my thumb. I tend to wear latex or nitrile gloves while cleaning the fish.
I cut from the anus to the gills, and pull everything out. I cut off the pectoral fins on a diagonal pointed towards the gills, and then I cut off the head and gills.
Don't throw away the heads, guts and fins. Stuff them in a freezer bag and hide them at the back of your freezer. They are quite useful as crab/crayfish bait, or, if you are a chef, the heads and fins make for good stock.
On a large enough fish, I leave the head on, as there is quite a bunch of meat in the head of a fish.
Same advice as before - if you want to remove the scales, and have a hose, just point the fish away from you and hit them with a few blasts of water - the scales are so loose that they'll basically just wash off. I just grab the fish by the head, stuff my thumb in the gillplate, and scrape the side of the fish with the outside edge of my kitchen shears.
If you really want to get technical, and use a world class tool that would be overkill for trout
1. get a bottle cap.
2. get a small nail
3. get a hammer
4. get a stick or a dowel or some such.
now, take the nail and hammer and punch a hole through the approximate center of the bottle cap from the inside of the cap. Next, take the nail and hammer, and hammer the nail through the hole that you just made in the bottle cap, from the inside. Using the nail/bottlecap combo, mark the approximate center of one side of one end of the stick. using this mark as a guide, nail the bottlecap to the stick, with the edges facing outward.
It should be pretty obvious how to utilize this.