you can start by choosing to render the fields manualy.
A simple example:-
{% for field in form %}
<div class="fieldWrapper">
{{ field.errors }}
{{ field.label_tag }} {{ field }}
{% if field.help_text %}
<p class="help">{{ field.help_text|safe }}</p>
{% endif %}
</div>
{% endfor %}
This will render the form without any styling then you have to apply the relevent class to create a drop down list(copied from w3c dropdown css):-
<div class="dropdown">
<button class="btn btn-default dropdown-toggle" type="button" id="menu1" data-toggle="dropdown">Tutorials
<span class="caret"></span></button>
<ul class="dropdown-menu" role="menu" aria-labelledby="menu1">
<li role="presentation"><a role="menuitem" tabindex="-1" href="#">HTML</a></li>
<li role="presentation"><a role="menuitem" tabindex="-1" href="#">CSS</a></li>
<li role="presentation"><a role="menuitem" tabindex="-1" href="#">JavaScript</a></li>
<li role="presentation" class="divider"></li>
<li role="presentation"><a role="menuitem" tabindex="-1" href="#">About Us</a></li>
</ul>
</div>
</div>
二者:
<div class="dropdown">
<button class="btn btn-default dropdown-toggle" type="button" id="menu1" data-toggle="dropdown">Days
<span class="caret"></span></button>
<div class="fieldWrapper">
<ul class="dropdown-menu" role="menu" aria-labelledby="menu1">
{% for field in form %}
<li class="fieldWrapper">
{{ field.errors }}
{{ field.label_tag }} {{ field }}
{% if field.help_text %}
<p class="help">{{ field.help_text|safe }}</p>
{% endif %}
</li>
{% endfor %}
</ul>
</div>
</div>