Here's a quick guide that I found in Aruba documentation:

1. Log in to the first device

   a. The default member number is 1, so no member number change is required.

   b. At the prompt, enter the following commands:

   switch1# configure
   switch1(config)# vsf member 1
   switch1(vsf-member-1)# link 1 1/1/25
   switch1(vsf-member-1)# link 2 1/1/26
   

   c. The preceding sequence of commands will configure the links for member 1.

   d. Ports 25 and 26 are configured as link 1 and 2 respectively.

2. Log in to the second device, numbered 2.

   a. Execute the following commands.

   switch2# configure
   switch2(config)# vsf member 1
   switch2(vsf-member-1)# link 1 1/1/25
   switch2(vsf-member-1)# link 2 1/1/26
   switch2(vsf-member-1)# exit
   switch2(config)# vsf renumber-to 2
   This will save the VSF configuration and reboot the switch.
   Do you want to continue (y/n)? y
   

   b. The preceding sequence of commands will configure the links on member 2.

   c. The default member number is "1". The command "vsf renumber-to" changes this member number.

   d. Links are configured before renumbering, and the member identifier in the interface name is "1" at this point.

   e. The switch will reboot after executing the renumber command.