You need to give more details. If the health bar only has values 0, 10, 20 up to 100 for example then a set of costumes might be enough.

@TheLogFather made one here: https://scratch.mit.edu/projects/18482376/

K-playgaming wrote:

when green flag clicked

Say you want ten health, make ten costumes (in the health bar sprite) with one less (or more) hp point per costume

Then make a variable call playerlives or bosslives or whatever and make this:

when green flag clicked
set [ playerLives] to [10]
forever
switch costume to ((circleLives) + (1))
end

Then when you remove health by using

change [ playerLives] by (-1)

It will go down