Step 1 : The first thing we’ll need to do is create two custom fields

a) Create a OptionSet field and populate it with some value say field is  “new_SelectOption”

b) Create a MultiLineTextBox to store string of values selected from the OptionSet say “new_SelectOptionValue”

Step 2 : Put the OptionSet Field on the Entity Form

Step 3 : Create a JavaScript web Resource and add the following sample code

onLoad Event :
         
          function ConvertToMultiSelectCheckBox( )
          {
            OSet=document.all.new_SelectOption; //get OptionSet field object
            OSetValue=document.all.new_SelectOptionValue;     //get MultiLineTextBox field object

            if( OSet != null && OSetValue != null )
            {
                OSet.style.display = "none";
                Xrm.Page.getControl("new_SelectOptionValue").setVisible(false);

//Hide the control
               
                // Create a DIV
                var addDiv = document.createElement("<div style='overflow-y:auto; height:80px; border:1px   #6699cc solid; background-color:#ffffff;'/>");
                OSet.parentNode.appendChild(addDiv);
     
                // Initialise checkbox controls
                for( var i = 1; i < OSet.options.length; i++ )
                {
                    var pOptionSet = OSet.options[i];
                    if( !IsChecked( pOptionSet.text , OSet, OSetValue) )
                    {
                         var addInput = document.createElement("<input type='checkbox' onclick='OnCheckChangeEvent()' style='border:none;width:25px; align:left;' />" );
                    }
                    else
                    {
                        var addInput = document.createElement("<input type='checkbox' onclick='OnCheckChangeEvent()' checked='checked' style='border:none; width:25px; align:left;' />" );
                    }     
                    var addLabel = document.createElement( "<label />");
                    addLabel.innerText = pOptionSet.text;
                    var addBr = document.createElement( "<br/>");
     
                    OSet.nextSibling.appendChild(addInput);
                    OSet.nextSibling.appendChild(addLabel);
                    OSet.nextSibling.appendChild(addBr);
                }
            }
        }

        function OnCheckChangeEvent()
        {
            OSet=document.all.new_SelectOption;
            var getInput = OSet.nextSibling.getElementsByTagName("input");
            var result = '';     
            for( var i = 0; i < getInput.length; i++ )
            {
                if( getInput[i].checked)
                {
                    //selected values are stored as semi-colon delimited values
                    result += getInput[i].nextSibling.innerText + ";";
                }
            }     

            //Save MultiLineTextBox data into the database
            control = Xrm.Page.getControl("new_SelectOptionValue");
            attribute = control.getAttribute();
            attribute.setValue(result);
        }

      // Check CheckBox is Checked or Not
        function IsChecked( pText , OSet, OSetValue)
        {
            if(OSetValue.value != "")
            {
                //selected values are stored as semi-colon delimited values
                var OSetValueT = OSetValue.value.split(";");
                for( var count = 0; count < OSetValueT.length; count++ )
                {
                    if( OSetValueT[count] == pText )
                    return true;
                }
            }
            return false;
        }