Peel the potatoes and cut into roughly 4cm chunks - try to keep the size of the chunks relatively similar so they have similar cook times.
1 kg potatoes
Add the potatoes to a pot and cover with water with 1 tsp salt. Place on the stove and bring to the boil. Once boiling, turn down to simmer until the potatoes are fully cooked through (15-20 minutes). You can test this by removing a chunk of potato from the water to a plate and trying to mash it with the back of a fork - if it mashes easily, it's done! If not, let them boil a bit more and check again in a couple of minutes.
Once the potatoes are cooked, drain well and return them to the pot.
Add the olive oil and crushed garlic to a small pan or pot over a medium heat on the stove. Once the garlic starts sizzling, let it cook for 30 seconds then remove from the heat.
4 tbsp olive oil, 3 cloves garlic
Pour the oil & garlic into the pot of hot potatoes and start mashing together with a potato masher, adding soy milk a splash at a time, until the potatoes are mostly smooth. Season generously with salt and pepper and mash until they've reached your desired consistency - you can add more soy milk if you think it needs to be wetter. If you're adding the marmite/vinegar (they give the mash a more buttery flavour), fold them in now.
100-150 ml unsweetened non-dairy milk, salt & pepper, 1 heaped tsp marmite, 1 tsp malt vinegar, white wine vinegar or apple cider vinegar