How to call Suitelet form from Custom button

1. Create a new suitelet script and deploy (check if it works correctly by clicking on the url)

Code:

function demoSimpleForm(request, response)
{
   if ( request.getMethod() == 'GET' )
   {
        var form = nlapiCreateForm('Simple Form');
        form.addField('textfield','text', 'Text');
        form.addField('datefield','date', 'Date');
        form.addSubmitButton('Submit');
        response.writePage( form );
   }
   else
        dumpResponse(request,response);
}

2. Create a new user event script to add button and deploy on the specific record

Code:

function beforeLoad_addButton(type, form) {
   form.addButton('custpage_buttonalert', 'custom Button', 'onclick_callClient()');  
   form.setScript('customscript444'); // where customscript444 is the client script id which you will create in step 3  
}

3. Create a client side script, and save it do not specify the function,we calls the function from user event script

Code:

function onclick_callClient(){
var recID = nlapiGetRecordId();

  var url = nlapiResolveURL('SUITELET', 'customscript442', 'customdeploy1')+'&entity=' + nlapiGetRecordId() +  '&recordType=' + nlapiGetRecordType();
// where customscript442 is the script id of the userevent script in step 2 and customdeploy1 is the deployment id in step 2
   document.location=url;
}