It is imperative that you use a high quality, espresso-capable burr grinder paired with freshly roasted, whole beans ground to size right before use.
The Signature's Bottomless 2-in-1 portafilter is "non-pressurized", which will allow you to craft true espresso, and therefore requires use of a very fine, consistent grind to build resistance in the brew head to create pressure. You must have an espresso-capable grinder to take your shot into your own hands and find the perfect size for your specific roast, dose, bean freshness, etc.
If you already have the Signature and do not wish to upgrade your grinder at this time, consider swapping in the Flow Control portafilter (as seen on the NEO) for "pressurized brewing" until then.
If you're looking for a grinder recommendation, we of course recommend our Royal Grinder - learn more here. If looking at alternatives, ensure it has been vetted for use with espresso, has high-quality burrs, and at least 60 adjustment settings. Most capable manual grinders will be $150+, with electric options being more.