Begin by looking at the relationship between the first letter of the first two pairs, C and E.
Using the alphabet line, you can see that by counting forward two letters, 'C' becomes 'E'
Now, by using the same rule, look at the first letter of the second two pairs, 'B' and count forward two letters. 'B' becomes 'D' and you have the first missing letter.
Next look at the relationship between the second letter of the first two pairs, G and D.
Using the alphabet line, you can see that by counting backward three letters, 'G' becomes 'D'
Now, by using the same rule, look at the second letter of the second two pairs, 'W' and count backward three letters. 'W' becomes 'T' and you have the second missing letter.