CIDR is the number of contiguous one bits in the IPv4 mask(32 bits) starting at the most significant bit.

10000000 00000000 00000000 00000000 where 1 = most significant bit

The common masks are /8, /16, /24 which all fall on an 8 bit(octet) boundary.

11111111 00000000 00000000 00000000 = /8 = 255.0.0.0

11111111 11111111 00000000 00000000 = /16 = 255.255.0.0

11111111 11111111 11111111 00000000 = /24 = 255.255.255.0

It isn't really hard when it is not octet aligned, but it does require some simple math and understanding of what an octet is.

11111111 11111111 11100000 00000000 = /19

The first two octets of the mask are 255.255 (/16 is less than /19). The last octet is 0(/19 is less than /24). So far we know

11111111 11111111 11100000 00000000 = /19 = 255.255.?.0

When looking at each octet remember it is an 8 bit value, 0 - 255.

0 0 0 0 0 0 0 0
1
2 6 3 1
8 4 2 6 8 4 2 1
So the third octet (?) 11100000 is 128 + 64 + 32 = 224. That means

11111111 11111111 11100000 00000000 = /19 = 255.255.224.0

11111111 11111111 11110000 00000000 = /20 = 255.255.240.0
(first 20 bits are taken above, leaving the remaining 12 available )
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To figure up the available hosts/ips subtract the CIDR from 32. For example /20 would be 32-20=12 which would be 2^12 power or 4096 IP minus 2 totaling 4094