How to say "it was nice meeting you yesterday"
Way to go! You have made a perfect first Chinese sentence "我昨天很高兴见到了你。"! 

In 我昨天很高兴见到, you might have to put a target for the verb '见到', so it would be like “我昨天很高兴见到你。”, which has the same meaning as "我昨天很高兴见到了你。" 

‘了’ in this case can be omitted.