Step by step tutorial: 
1) prepare your scene

add grass models and open addon tab by pressing "N", addon locates in "Tool" category

2) SCATTER workflow

select your ground object and press "Add Physical Scatter"

Effect settings are located in modifiers tab

Press "Create Collections" and it will add named collections to your scene

Organize your objects into collections

Move collections to modifier settings 

This effect uses 5-layer scatter system 
1 layer - Cover Grass

2 layer - Medium Grass 

3 layer - Tall Grass

4 layer - Flowers

5 layer - Misc

Create vertex button will add 5 vertex groups to your mesh

Copy their names into layers names! They are case sensitive - so double click on vertex group name to select all and copy.

You can make your own names 

And set your vertex groups in edit mode or in weight paint mode

Now set up density and scale

Physics Settings

Less value=more area got affected by collision. Bigger value = smaller area\

Defines How fast grass will restore to its initial position. 1=fast. 0.001 = very slow

dont change this value

Use this settings to change collisions form settings. Collision Strength: 0.02 = smooth collision interaction, 1=very fast interaction. Max and min windth affects collision range. I strongly recommend use these setting to set up collisions.

WIND
- wind/force strength

- turbulence settings. Works like noise texture

- make this mask more contrast.

wind and wind noise

-Experimental Function. Affects perfomance

Gradient Scaling

Create new vertex groups and if enabled - it will allow you to draw grass length, based on weight paint

Fix Collisions 
 

Deleted grass inside object if going through geometry. If strange behaviour reduce adjust form value. If doesnt work - make this value bigger, up to 100

3) Grass Draw

- Add Curve - appends curve object at the location of your 3D cursor. After appending - press Grass PhysX Curve. It will append modifier to your curve.

To paint curve go to edit mode - brush tool - surface

into target collection put your ground mesh

IMPORTANT!!!

IF EFFECT LOOKS BUGGY - APPLY SCALE TO YOUR OBJECTS (ctrl+a). 

FIRSTLY TRY TO APPLY SCALE TO YOUR GROUND/CURVE, SECONDLY, IF FIRST STEP DIDNT CHANGE ANYTHING, GRASS INSTANCES SCALE

APPLIED SCALE