main:    
        # create the first node 
        li      $v0,9             # allocate memory
        li      $a0,8             # 8 bytes
        syscall                   # $v0 <-- address
        move    $s1,$v0           # $s1 = first node
        
        # copy the pointer to first
        sw      $s1,first
        
        # initialize the first node
        li      $t0,1             # store 1
        sw      $t0,0($s1)        # at displacement 0

Second Node

Now look at the second node. Rather than use a second register for the second node $s1 is used again:

        # create the second node 
        li      $v0,9             # allocate memory
        li      $a0,8             # 8 bytes
        syscall                   # $v0 <-- address
        
        # link the second node to the first
        sw      ___,4($s1)        # copy address of second node
                                  # into the first node
        
        # make the new node the current node
        move    $s1,___           # $s1 = second node

        # initialize the second node
        li      $t0,2             # store 2
        sw      $t0,0($s1)        # at displacement 0