You need to take a *single* constructor to an *empty* square and then select the construct option. It's a little button to the left of the details of the constructor. Like where guard and sentry commands are.
Although I must warn you, Influence starbases will not change how other races feel about you, they effect your borders and influence flipping enemy planets. Putting an Influence starbase next to another races planets may actually cause them to not like you and even declare war.
