Take the binary number and divide it into groups of 4 bits starting from the right hand side , if no group of 4 exists fill out leading 0's, then convert each group of 4 to a hex number directly e.g. 
 
      1111110₂ -> hex direct
      111   1110 add leading zero
      0111   1110
 
      111  -> 7_H
 
      1110 -> 1410 ->E_H
 
      1111110₂ -> 7E_H
 
This is easily achieved in reverse by converting hex straight to binary i.e. divide the hex number into single hex digits from the right hand side and convert to binary each digit e.g.
 
    7EH -> binary direct
 
        7 -> 111₂
 
        E -> 1410 ->1110₂
 
        7EH -> 1111110₂