There are few dishes simpler than Shepherd's Pie. As with all simple recipes there are countless variations, feel free to share your own in the comments below (bonus points for pictures!)

There's some interesting learning on the topic if you want to enlarge your brain before eating.
"In early cookery books, the dish was a means of using leftover roasted meat of any kind, and the pie dish was lined with mashed potato as well as having a mashed potato crust on top"(Wiki)

Feel free to substitute any or all of the ingredients listed to make your own variety.

Enough talk, let's mash!

Step 1: Ingredients

Here's what I used, but you can substitute as needed. I wanted a recipe that was dead simple and used some very common ingredients. If you want to make this dish your own I've suggested some alternatives in brackets beside what I used:

Tater-Gems (any type of homestyle fry/chip/crispy potato)
Ground beef (lamb/ground round)
Corn & onion (peas/carrots/celery)
2 large potatoes (yams/turnips)

Step 2: Pre-heat

Yes, this requires it own step as warming your oven is often overlooked.
Go do it now, I'll wait.

While the oven is heating up we'll be prepping the remaining ingredients, by the time we're finished the prep the oven will be ready!

Step 3: Cook Potatoes (and Tots)

I don't own a microwave because I have an inane fear that I'll irradiate my face when looking at my food behind that thin plastic door. Instead I cooked my potatoes the old fashion way, by boiling them!

Fill pot with water and place on stove to boil
Peel potatoes (or don't)
Rinse potatoes
Cut potatoes*
Plunk into boiling water
Reduce heat to a medium boil
Let cook until soft (about 10-15 minutes)

*Smaller cubes cook faster.

By now your oven should be heated and ready to receive your tots. Place tots on a cookie sheet evenly spaced and fire them into the oven.

Step 4:

While everything is cooking we can start on the filling.

Start by chopping up some onions and get them into a hot frying pan, cook until nice and caramelized. Once the onions have turned a nice golden brown/black add in your raw ground beef and cook until brown. Add some cracked pepper or other seasoning as desired.

Step 5: Drain Canned Goods

While you're cooking the beef and onions it's time to drain the corn.
I used canned corn but you can use whatever you have, fresh corn (no cob), creamed corn, fresh cut veggies, or frozen veggies.
This dish is accepting to a wide interpretation, so be creative!

A neat trick for draining liquids from canned goods is to make 2 openings in the lid before opening fully. I used my can opener to make a small opening on one side and a slightly larger opening on the other, then tipped the entire can upside down into the sink to drain.

The lid acts as a strainer and keeps my hands free to check on things while it's draining.

Step 6: Add Corn to Meat

After liquid has been drained open can fully.
Turn off heat to meat and add corn.
Mix and let sit until we combine all our ingredients.

Step 7: Mash!

Making mashed potatoes is easy.
Once potatoes are cooked (soft when poked with fork or skewer, usually 20 minutes), drain and place into a bowl. Add a gob of butter and a dash of milk and get mashing. Mash until desired consistency is reached.
You can add more salt, butter, sour cream, or milk to your taste.

If you prefer, here's some instructables that take mashed potatoes to a science!

Step 8: Bringing It All Together

By now our meat, tater tots, and potatoes are all done. It's time to assemble the pie.

Start with a layer of tots, use your masher to make a flat bottom.
Add the meat filling, scoop meat out of pan to avoid dumping in excess fat.
Next, pile on your mashed potatoes.
Top with cheese! ^{(that's right, I said cheese! Load it up!)}

Step 9: Bake

Since the components of this dish have been cooked separately this step is to bring cohesion to the ingredients and get them piping hot for dinner time. It also allows that great browning effect on the top of your pie, something that people will ogle and salivate over.

After about 10 minutes the top should have browned, this means you're done!
Remove from oven and let rest for a few minutes before serving.

Enjoy!