When I'm back from Europe, how do I avoid SG custom GST?

You don't. It is illegal and considered tax evasion.

Their checks are random. If you kena and never declared then be prepared to face consequences.

However, there are a few tricks. You can remove all tags, remove packaging, use the item. That way, you did not buy it, you just brought it on your trip with you.
If you bought consumables like many bottles of wine, then this case you better declare. But if it is 1 or 2 handbag, you can "Use" it. If you buy 20 handbags, you better declare also...