Montessori math curriculum is generally introduced in the following order: (1) Number Rods (the first material to introduce to a child at about two years of age), then (2) Sandpaper Numbers, (3) Spindle Box, (4) Numbers Memory Game, followed by Odd and Even activity (also called cards and counters). You can use a traditional Montessori Numerals and Counters (like here) or you can make it yourself (you would need numbers 1-10, and 55 round counters (you can use marbles, wooden dots etc). The activity teaches obviously what odd and even number is.
Presentation: place numbers in random order on the rug since you want to make sure that your child knows numbers one through ten before introducing a concept of odd/even. You would then ask your child to set numbers from one through ten horizontally at the top of the rug.
You would position counters in pairs, making it obvious for your child to see when a number has a counter without a pair (on its own)- meaning that number is odd. Versus when all counters have a complete set of pairs (two friends are holding hands) that number is even.
I would suggest introducing this material not earlier than three years of age, since although concrete, this activity still introduces a very abstract concept of odd vs even. Adrian just turned 38 months, and the concept of odd and even numbers was confusing, even with all the visual counters, so I will wait a month and re-introduce it later.