I want to run D20 but all I have are a couple of D6s

We've all been there. It's Christmas and you're gathered at Auntie Doreen's place where your siblings and cousins want you to run one of your 'Dumbledores and Dragons' games for the kids to keep them occupied. You didn't bring your gaming dice and Doreen is on dial-up because wifi causes autism and fibre is a plot by animal rights activists to get their propaganda into people's homes. All you've got to work with are a couple of D6s from an old copy of Monopoly that has POO BUM scrawled across the board in crayon. You can do this.

You can use D6s for a fair simulation of other dice. The methods are neither elegant nor straightforward, but they do what they need to do. In the list of operations below I'm going to call the first die DA and the second DB.

D4

The easiest of them all: just chuck DA and re-roll if you get a result higher than 4. You'll probably have to roll three times for every two results you need, but there's no maths involved.

D8

Roll both dice. DA is a D4. If DB shows an odd side, add 4 to DA's number to get your D8 result.

D10

Roll both dice. This time you're rolling DA as a D5 (re-roll on a 6 result). DB is even/odd again. If it shows an odd side, add 5 to DA's number for your D10 result.

D12

Do not roll both dice and add them together. The first issue is that it's impossible to get a 1 result that way. The second is that rolling two dice and adding them gives you a weak bell-shaped probability curve instead of the flat result you want. The odds of rolling a 6 this way are 5/36 (14%) but the odds of rolling a 12 are 1/36 (0.28%).

Instead, roll DA as a D6 and DB as an odd/even to determine if you add 6 or 0 to DA.

D20

Now it's getting kind of awkward. Roll DA as a D5. Roll DB as a D4. You want a result of 0 to 3. For simplicity's sake I like to treat 6 as a 0, take 1 to 3 as rolled and re-roll on a 4 or 5. Add DB x 5 to DA to get a D20 result.

D100

Use the D10 method twice, once for the tens and again for the ones. Subtract 1 from each result. This will give you final result of 00 - 99. You can treat 00 as 100. If you leave out the subtractions, the lowest result you can roll is 11.

Doreen won't mind you using her toby mug collection as giant figurines, will she?