Then try adding in some little flirty details and compliments. If they return these then just ask them out. Try and bring the conversation round to an appropriate point first. For example, start talking about films and then ask them on a cinema date.
Your natural body language will help you because you will subconsciously be trying to indicate that you like them and your body should react to this.
It's always slightly nervewracking but why not give it a go.
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