(Suuuuper optional! It's obv stronger to roll&place normally!)

Roll 1d10 and 1d12.

Put a 14 in constitution.

There are five empty spots remaining.

Read the d10 as if it were a 1234512345 die (i.e. subtract five if it's over five). So if you rolled a seven, that's the second empty spot, dex. Put 15 there.

There are four empty spots remaining.

Read the d12 as if it were a 123412341234 die. (i.e. keep subtracting four as long as it's over four.) So if you rolled an 8, that's the fourth empty spot. Put a 13 there.

There are three empty spots remaining.

Read the d12 as if it were a 111122223333 die. (I.e. divide it by four and round up.) So if you rolled an 11, that's the third empty spot. Put a 12 there.

There are two empty spots remaining.

Read the d10 as if it were a 1111122222 die. (I.e. as a normal "high/low" die.) So if you rolled a 4, that's the first empty spot. Put a 10 there.

Put an 8 in the last empty spot. Yer done!