Auto Flowers are on a timer from the time they crack until they flower so getting it right when they are very young really matters with autos.
They come out of the ground hungry so you need to feed them right away. And not over water them. High light levels help but getting it dialed in so you don't stunt them may take a run or 2. Plant cracked seed in a big pot never transplant. Auto Flowers like a very light and fluffy soil.
I use 50/50 Peat Moss and Vermiculite.
Mix in
40 grams of Dolomite.
10 grams Gypsum
Per gallon medium.
This is Calcium, Magnesium, Iron and a few other goodies.
Find a simple liquid fertilizer.
Miracle Grow liquid in the yellow bottle 12-4-8 works real well.
5ml per 3 Litres water = 200 PPM nitrogen to start with.
When she starts to flower 3rd week switch to something like the Miracle Grow green bottle 4-12-4 for flowering.
This is a type of hydroponic grow so start off week 1 just enough solution to keep the seedling alive.
Week 2 a little more solution. Never just water. Week 3 is time to switch.
Now you must add enough solution that you get 25% runoff every other day. This flushes old nutrient salts and re introduces fresh fertilizer at the same time. Your seedling top has been growing slowly on top while growing it's root system underground for the last 3 weeks and will take off and grow like crazy week 3 and beyond.
I mix 10 ml per gallon water in a jug and PH 6.5 Flush water every other day threw veg and stretch.
Switch to a blooming mix like 4-12-4 same 10ml per gallon. Same watering.
Let her flower.
You can add a gram of Monster bloom 0-50-30 to the gallon of 4-12-4 if you want to add a bloom booster.
It's about $20 a bottle on Amazon.
Try this and see.
A few guys will knock the Miracle Grow but it works very well.
It's affordable and easy to get your hands on.