Can I use 完 after any verb to indicate the completion of activities? Is it okay if I want to say 做完 (finish working)? I have come across 吃完 (finish eating). Is it possible to say finish [doing bla bla] by writing ...完?
... can be filled with 做 or 写, etc...
Yeah totally.
KEY

finish (the action of the preceding verb)

Tuttle Learners

finish, end
电影什么时候完？
Diànyǐng shénme shíhou wán?
When will the movie end?
吃完
chīwán
finish eating, eat up
我吃完饭就去开会。
Wǒ chīwán fàn jiù qù kāihuì.
I'm going to a meeting as soon as I finish my meal.
看完
kànwán
finish reading/watching
我昨天看完电视已经十二点了。
Wǒ zuótiān kànwán diànshì yǐjīng shí'èr diǎn le.
It was already twelve o'clock when I finished watching TV last night.
做完
zuòwán
finish doing
你什么时候可以做完作业？
Nǐ shénme shíhou kěyǐ zuòwán zuòyè?
When can you finish your homework?
用完
yòngwán
use up
我的钱用完了, 我要到银行去取钱。
Wǒ de qián yòngwán le, wǒ yào dào yínháng qu qǔ qián.
I've used up my money. I'll go to the bank to get some cash.

Just add it to the end of any verb.