Actually, it is perfectly correct. The topic here is ONE process, not a number of processes. Consider: "(The process of) Repeating key words and using time-sequencing words is commonly done when describing a (different) process." If the topic was referring to TWO processes, then the plural would be used. But then the sentence would change: "BOTH repeating key words AND using time-sequencing words ARE commonly done when describing a process."

I agree with Randy.

No, it is incorrect. The plural should be used. “Repeating key words and using time-sequencing words” represents a compound subject consisting of two gerund phrases. (A gerund is the -ing form of a verb used as a noun.)