I’m going to be a double major in Statistics and Econ at UNC - Chapel Hill.
Get great grades. See if you can get some research experience with a prof on campus. If there is an opportunity to do a senior thesis, do it. Build relationships with your professors – you likely will need 3 strong recommendations. Study for and nail the GRE when you take it.
And take math classes; I’m not in econ, but have learned from other people that intermediate to advanced math makes you more competitive for math PhD programs. Most successful PhDs in econ (as I have heard) have three semesters of calculus, 1-2 semesters of statistics and probability and potentially a semester of differential equations and real analysis. You have to take most of that as an undergraduate statistics major anyway.
Talk to your economics professors, once you get comfortable with a few. Especially if you are at a university with a PhD program, they can give you some insight into what PhD programs look for in students - but even if you aren’t, they can share the general profiles of prior students who have been accepted into PhD programs in econ.
I would actually go further than juillet and say that nowadays, to maximise your competitiveness for a top PhD econ program, you should complete at least a math minor and perhaps even a full math major, and not even an applied math major, but a true math major. The particular math course sequence you take should be as rigorous and proof-based as possible. {Some schools provide math sequences where you can complete the major while doing relatively few proofs: I would urge you not to consider this.}
If necessary, I would say to swap the econ major for a math major. One of the great ironies of academia is that a math major is actually more competitive for admission to an econ PhD program than is an econ undergrad major.