Make sure you don't go 1v5 without your team aside. Even when you are fed, you can get stunned or lose your R. Play more carefully. Be careful in early, Katarina won't be so powerful when she loses early game, so if you have a hard match up, just play deffensive.
The best moment for roaming is when you did a preasure on Mid, pushed bigger minion wave to the tower and killed the enemy mid. I recommend having lvl 6 when roaming, so you can quickly do the damage, get kill/assist and come back to your lane

The most common Combo is Q-E-W-E and R in between.
I recommend getting to know with much better one
E-W-Q-E. You jump into the enemy and throw W, you catch the blade and throw Q, and jump into the blade again. You can finish that combo with R if needed, or in team fights to do as much damage as you can.
You have to remember that catching a blade resets your E and every assist/kill shortenes your R cooldown. It is very useful to know these things, so you can really adapt to the situation.