To configure the generator, press on the gear on the left, at the top. Here you can change the time between the ticks in the generator and the size of the boxes in the generator.
To start, press space. Enter the start height of the particles (changeable with v and n later in drawing mode), the name of your particle, the volume of the particle-cloud, the speed value of the particles,
the number of particles per tick and finally the tag, the entity executing the effect will have.
3.. 2.. 1.. Go! Now you can begin to draw. Press b to draw with no particles, press it again and there they are again. Space to end drawing mode.
Last step, copy the text and paste it into the text-field beneath. Click finish and copy everything. Now paste it into a .mcfunction and enter ingame /gamerule gameLoopFunktion [yourFunctionName]
You've done it! Have fun with it!