Use fdisk, gdisk, or parted to create a partition for swap. For example, following are the steps to create a swap partition on /dev/sdb using fdisk.

Determine swap space

Prior to adding swap, the free command can be used to view the current swap space. In this example, 

~]# free -h
        Total    Used    Free
. . .
Swap:    500MB   200MB   300MB

Create a new partition

Type fdisk /dev/sdb and press enter.

[root@server1 ~]# fdisk /dev/sdb
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): 

Type n (new) and press enter.

Command (m for help): n
Partition type:
  p   primary (1 primary, 0 extended, 3 free)
  e   extended
Select (default p):

Type p (primary partition) press enter.

Select (default p): p
Partition number (2-4, default 2):

When prompted Partition number, press enter to use the default partition number

Partition number (2-4, default 2):

When prompted First sector, press enter to use the default first sector

When prompted Last sector, press enter to use the default last sector

Type w and press enter to write the changes

To set the type

Type fdisk /dev/sdb2 and press enter.

[root@server1 ~]# fdisk /dev/sdb1
Welcome to fdisk (util-linux 2.23.2).

Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): 

Type t (type) and press enter.

Command (m for help): t
Selected partition 1
Hex code (type L to list all codes): 

Type 82 (swap) and press enter.

Hex code (type L to list all codes): 82
Changed type of partition to 'Swap'

Command (m for help): 

Type w and press enter to write the changes.

Command (m for help): w

Create a swap file system

Use the mkswap command to create a swap file system.

~]# mkswap /dev/sdb2

Turn on swap.

~]# swapon /dev/sdb2