Christians, can you explain the fertilisation process of an immaculate conception?

Technically it can happen, virginity is defined as having an intact hymen. If a man only put the tip of his penis in, without breaking the hymen and ejaculated then the semen could enter the womb and impregnate the woman. Also there is a small chance that ejaculating on the outside of the vagina can allow semen to pass in. Both of these cases are very rare however.