Depending on the LED series resistance chosen the circuit should be able to last 1 year on a single CR2032. Gerber files for the PCB included in the files, schematic/BoM included in the images. All told, it should cost less than $5 to make one (including PCB, components, and battery). Meant to be used as a last-resort reminder to water plants, not as an exact moisture level indicator!
Send the PCB's gerber files to any PCB manufacturer (JLCPCB, PCBWay, etc.) to have the PCB made and shipped to you. Should come out to around $8 for 5 PCBs.
Buy components to populate PCB
Need 2N7002 SOT23 (x3), 2M 0603 resistors (x2), 9.1M resistor (x1), 0603 red LED (x1), CR2032 surface mount battery clip (x1), 1N4148 SOD123 diode (x1), 47k 0603 resistor (x1), 2.2k 0603 resistor (x1), 680pF 0603 ceramic capacitor (x1), 80pF 0603 ceramic capacitor [OPTIONAL] (x1). All components should be easy to find on digikey and most from even Amazon.
Use a solder iron with a fine head to solder the components onto the PCB. Recommend using fine tweezers for soldering the 0603 components (and the 0606 flashing LED) and lead free solder. Please make sure to solder in a well ventilated area or using a fume extractor! NOTE: For best reliable results it is recommended that you apply conformal coating to the top side components of the PCB (the side with the LED, transistors, etc) in order to avoid inaccurate readings that can result if these components get wet.