A tonal scale is a really useful tool. It's simple to make and use and it's creation is a good visual exercise too. You can make it with any medium.
 

• Take a strip of white paper and divide it into five squares with no borders. 

• Number the squares 1,2,3,4 and 5.

• Square number 1 stays paper white.

• Fill in square 5 with solid black - or as dark as you can achieve with your medium of choice.

• Using squares 1 and 5 for reference use your visual judgement to find the mid tone between the two and fill in square 3.

• Using squares 1 and 3 for reference fill in square 2 with the tone that falls exactly between them.

• Repeat the process using squares 3 and 5 to complete square 4.