1) Decide which sensors to use. You can use 8 LM35's (or similar sensors) which would require 8-channel ADC. You can use 8 DS18S20's (or similar sensors) which would require 1-wire communication. You can use SPI-based sensors or I2C-based sensors. It's up to you to decide what to use and to make sure that you know how you'll interface it to the microcontroller. If you don't know already, learn how to interface to the LED display.
2) Go through the datasheet and application notes, reference manuals, design tips, etc of the sensor(s) you're going to use. Go through examples found on the internet of sensor interfacing, LED interfacing, etc.
3) Starting from scratch, learning everything, writing the code, designing the circuit, making the PCB, testing the circuit and finalizing it could take between a day to a month, depending on what you already know and how much new stuff you need to learn. I would say that if you can make it within a week, you're good. :smile: Two weeks, not bad.
Hope this helps.
Tahmid.