No need to join other BIG classes and waste time in traveling. Use internet efficiently and practice answer writing at your home whenever you are free, preferably atleast 1 Q&A per day.

DETAILED ANSWER:

When should I start answer writing?
How should I practice answer writing?

WHEN-

The earlier the better.
However during early days of preparation, one may neither have the knowledge nor the techniques to write a good answer.
You should remember that the confidence of writing good answers comes only by writing more and more answers. Some things are best learnt when you actually start doing them and learn from your mistakes.
That being said, as an aspirant, there is always this tendency to delay answer writing.
The imaginary moment when you are done with all your revision and ready for answer writing will never come

( Many a times , not even during the Main exam!)
Then what is the time when you start your Answer writing practice?
The best time is:
When you are 2-3 months into your preparation and have read Economy, Polity, History, Geography basic books for Prelims

OR

1 month after Prelims after you have revised a few important books for Mains. Start answer writing practice simultaneously.
Since you have read History, Geography basic books- you can attempt most of GS1 questions (History, Geography, Art and culture)
If you are regular with newspaper/Monthly magazines, you can solve GS2,3
Practice GS4 since the beginning, its mostly about your real life experiences and your own thoughts.

HOW-

Let me give you a rough idea how you can go about it...
Answer writing practice consists of KNOWLEDGE PART and the PRACTICE PART
I'll answer how to efficiently gain relevant knowledge separately else it will be a very long answer...
Coming to the answer writing PRACTICE PART especially for Current Affairs, since most of the Mains paper is based on Current affairs and how you connect it to the Static part:

2 Sources -

InsightsonIndia (if your are practicing Daily) AND/OR
IASBaba (if you wish to practice few days a month)
( follow just 1 if you are short on time)

STEPS: 7 DAYS LAG

Keep a gap of 7 days between reading newspaper and solving questions on that newspaper.
There's no point reading an article in the morning and writing an answer on that article in the evening.
Keep a gap of approx 7 days.
This will make you think hard on the topic you've read a week back while writing answers. You'll find it difficult to recollect information.
That's what happens in the actual exam!
This answer writing and checking will act as a revision.
It will also make you more serious while reading newspaper coz you are supposed to write answers based on it after a week. You'll focus more on the keywords.
1 week gap will allow others to write their answers to the question and the moderators to write an ideal answer to the question. You can then easily compare your answer with theirs.
Also many a time the same issue drags on over a week, so you will have more points to write in your answer.