How to convert Binary to Octal and Hexadecimal and vice-versa

Binary to Octal: Make the pair to 3 bits from the left are write its demical equivalent. For Binary to Hexadecimal: Make the pair of 4 bits are write its hexadecimal. Octal to Binary: For each digit in octal write its binary equivalent. Similarly for Hexa. to Octa do same for 4 digits. Thanks.