After single letters come words. Every word here can be typed with just the eight home keys of the home row, so no finger has to leave its place.
salad flask alfalfa
A word is harder than a string of letters, because the fingers have to move in the order of the word's letters and the rhythm slips more easily. When a mistake lands in the middle of a word, do not fix it in a hurry. Slow down and type the letter you got wrong calmly and correctly.
The space bar is pressed with a thumb, usually the thumb of whichever hand is free. The thumb hovers over the bar while the other fingers stay on their home keys.
Once these words come without thinking, the home row is yours and it is time for the two extra index finger keys.