The best time to buy your eSIM is a day or two before you leave home, while you still have steady WiFi. You buy the plan online, get a QR code by email, and load it onto your phone. Nothing switches on yet, so you will not waste any data. It just sits ready to go.
Pick a plan that matches how long you will be in Liberia. The 1 GB plan runs for 7 days and costs $6.02. That suits a short work trip or a traveler who mostly uses messaging, maps, and a few emails. If you are staying about two weeks, or you like to upload photos and use video calls, the 3 GB plan for 15 days at $16.06 is the better fit. Light users heading in for a few days can start with the cheapest option at $6.02 and top up later if needed.
Before you pack, check that your phone can use an eSIM. Most iPhones from the XR and XS onward work fine, and so do recent Google Pixel and Samsung Galaxy models. Your phone also needs to be unlocked by your home carrier. Do this check at home, not at the airport.