If you do not trust to any software random number generator due possible bugs or backdoors, you can generate a mnemonic code using dice yourself. Critical to security is the use of random numbers. Do not try to select words from the mnemonic dictionary in other ways. Even if it seems to you that you are doing this in an unpredictable way, most likely you are mistaken.
In the proposed generation scheme, it is most convenient to use 5 cubes at a time. If you have only 1 die, you should make 5 consecutive throws to generate 1 word. With manual code generation, it is not possible to calculate the checksum, which is written at the end in accordance with BIP39. If the wallet in which you plan to use your mnemonic code requires the correct code checksum, then you should adjust last word to correct checksum with mnemonic code loader tool.
You should select 12 / 15 / 18 / 21 or 24 (recommended) words using dice rolls and dice wordlist. If your throw result is not in the list, just ignore and continue. Some combinations are excluded for uniform probability distribution.