//set [block-name]
Once you’ve made a selection, you can type command this followed by the block you want, and it will fill in your selection with that block:
Example:
//set red_wool
You can also list more than one block, and it will fill it in with a mix, like this:
//set red_wool,red_terracotta
Note there are no spaces between items, just the comma.
And you can specify percentages, like this:
//set 80%red_wool,20%red_terracotta
If the percentages do not add up to 100, the command will calculate the percentages based on the total. For example, if you type 10%blue_wool,30%red_wool, you will get a 1:3 ratio, or 25% blue wool and 75% red wool.