Initially, the students will draw a rectangular grid and then divide it up into four equal sections. My grid was 20cm by 16cm, but these dimensions don't really matter - it could be bigger or smaller.
Then students will make four observational sketches of their cactus, one in each of the grid's four sections. Each sketch should be made from a different angle.
Once students are happy with their pencil sketches, they can go over their lines in fineliner (I used a Derwent Graphik 0.05). After this, all pencil lines (including the original grid) can be erased.
To finish, students can add hatching, cross-hatching and additional line weight to their drawing.