The --script-args flag is used to set the arguments of NSE scripts. For example, if you would like to set the useragent HTTP library argument, you would use the following:

$ nmap --script http-title --script-args http.useragent="Mozilla 999" <target>

You can also use aliases when setting the arguments for NSE scripts. For example, you have the following code:

$ nmap -p80 --script http-trace --script-args path <target>

Instead of the preceding code, you can use the following one:

$ nmap -p80 --script http-trace --script-args http-trace.path <target>

Users may select specific scripts when scanning using the Nmap option --script <filename or path/folder/category/expression>:

$nmap --script <filename or path/folder/category/expression> <target>

For example, the command to run the NSE script dns-brute is as follows:

$nmap --script dns-brute <target>

The Nmap Scripting Engine also supports the execution of multiple scripts simultaneously:

$ nmap --script http-headers,http-title scanme.nmap.org
   Nmap scan report for scanme.nmap.org (74.207.244.221) 
   Host is up (0.096s latency). 
   Not shown: 995 closed ports 
   PORT     STATE    SERVICE 
   22/tcp   open     ssh 
   25/tcp   filtered smtp 
   80/tcp   open     http 
   | http-headers: 
   |   Date: Mon, 24 Oct 2011 07:12:09 GMT 
   |   Server: Apache/2.2.14 (Ubuntu) 
   |   Accept-Ranges: bytes 
   |   Vary: Accept-Encoding 
   |   Connection: close 
   |   Content-Type: text/html 
   |   
   |_  (Request type: HEAD) 
   |_http-title: Go ahead and ScanMe! 
   646/tcp  filtered ldp 
   9929/tcp open     nping-echo  

In addition, NSE scripts can be selected by category, expression, or folder:

$ nmap -sV --script vuln <target>

$ nmap -sV --script="version,discovery" <target>

$ nmap -sV --script "not exploit" <target>

$ nmap -sV --script "(http-*) and not(http-slowloris or http-brute)" <target>

Expressions are very handy as they allow fine-grained script selection, as shown in the preceding example.

To debug NSE scripts, use --script-trace. This enables a stack trace of the executed script to help you debug the script execution. Remember that sometimes you may need to increase the debugging level with the -d[1-9] flag to get to the bottom of the problem:

$ nmap -sC --script-trace <target>
$ nmap --script http-headers --script-trace scanme.nmap.org
   NSOCK INFO [18.7370s] nsock_trace_handler_callback(): Callback:   
   CONNECT SUCCESS for EID 8 [45.33.32.156:80] 
   NSE: TCP 192.168.0.5:47478 > 45.33.32.156:80 | CONNECT 
   NSE: TCP 192.168.0.5:47478 > 45.33.32.156:80 | 00000000: 
   48 45 41 44 20 2f 20 48 54 54 50 2f 31 2e 31 0d HEAD / HTTP/1.1  
   00000010: 0a 43 6f 6e 6e 65 63 74 69 6f 6e 3a 20 63 6c 6f     
   Connection: clo 
   00000020: 73 65 0d 0a 55 73 65 72 2d 41 67 65 6e 74 3a 20 se  
   User- Agent:  
   00000030: 4d 6f 7a 69 6c 6c 61 2f 35 2e 30 20 28 63 6f 6d   
   Mozilla/5.0 (com 
   00000040: 70 61 74 69 62 6c 65 3b 20 4e 6d 61 70 20 53 63 patible;   
   Nmap Sc 
   00000050: 72 69 70 74 69 6e 67 20 45 6e 67 69 6e 65 3b 20 ripting   
   Engine;  
   00000060: 68 74 74 70 73 3a 2f 2f 6e 6d 61 70 2e 6f 72 67   
   https://nmap.org 
   00000070: 2f 62 6f 6f 6b 2f 6e 73 65 2e 68 74 6d 6c 29 0d       
   /book/nse.html)  
   00000080: 0a 48 6f 73 74 3a 20 73 63 61 6e 6d 65 2e 6e 6d  Host: 
   scanme.nm 
   00000090: 61 70 2e 6f 72 67 0d 0a 0d 0a                   ap.org     
   [Output removed to save space]Nmap scan report for scanme.nmap.org   
   (45.33.32.156) 
   Host is up (0.14s latency). 
   Other addresses for scanme.nmap.org (not scanned):    
   2600:3c01::f03c:91ff:fe18:bb2f 
   Not shown: 995 closed ports 
   PORT      STATE    SERVICE 
   22/tcp    open     ssh 
   25/tcp    filtered smtp 
   80/tcp    open     http 
   | http-headers:  
   |   Date: Sun, 24 Apr 2016 19:52:13 GMT 
   |   Server: Apache/2.4.7 (Ubuntu) 
   |   Accept-Ranges: bytes 
   |   Vary: Accept-Encoding 
   |   Connection: close 
   |   Content-Type: text/html 
   |    
   |_  (Request type: HEAD) 
   9929/tcp  open     nping-echo 
   31337/tcp open     Elite 

   Nmap done: 1 IP address (1 host up) scanned in 18.89 seconds 

There will be occasions where you will want to try scripts not included officially with Nmap. To test new scripts, you simply need to copy them to your /scripts inside your Nmap directory and run the following command to update the script database:

# nmap --script-updatedb

After updating the script database, you simply need to select them, as you would normally do with the --script option. In addition, you may execute scripts without including them in the database by setting a relative or absolute script path as the argument:

# nmap --script /root/loot/nonofficial.nse <target>

The https://secwiki.org/w/Nmap/External_Script_Library Wiki page attempts to keep track of all scripts that for different reasons could not get included officially with Nmap. I recommend you visit it as there are some great scripts in there.