Get yourself some peeled potatoes and chop them into roast sized pieces. Then boil them until they are under done. After that, chuck them into a hot pan with loads of oil (or goose fat) and some flavours. I like using loads of garlic (whole and crushed) and cinnamon sticks for a really fantastic aroma and taste. After about half way through cooking in the oven, take a potato masher, or slotted spoon and break all of the potatoes, adding more oil and any herbs, pepper etc. Give them a good mix about. When you are ready to serve, remove anything unedible (like the cinnamon sticks) and then just scoop the rest out.
From Frozen
Sometimes there isn't time to make roasts from scratch. Maybe you've not got room on the hob to boil them (true story) or maybe you went to peel the potatoes to find they were on the iffy side (another true story) or maybe you just forgot and left it too late (this true story more often than not). But it doesn't matter. Because just do exactly the same thing to your frozen ones as you would home made ones. Put them in with oil and flavour, and then half way through, mash and break them up, adding more oil, pepper, herbs. I have done this many times and no one ever notices that they are not made from scratch because they taste that good. Genius!