Have just opened an IBKR account. Are there any guides that are available to follow to change SGD to USD to purchase IWDA ETF?

There's a 'Convert Currency' function available now on the IBKR mobile app, which does exactly what you you need. Just select SGD as the currency you have and USD as the currency you want, then slide to convert.

I don't know about the factual currency conversion fees of IBKR effected when you buy USD denominated shares with SGD, You usally don't see the fees for these conversions, when Your broker does this conversion during the Buy/Sell.

I am sure that Transferwise would do it cheaper, You can open an account with them

and they are safe, I feel, everything went well, when I transferred funds.

f.ex. for 5000 SGD converted to USD (and sent to Your account) they charge only 0.46%, which is much less than the quite invisible fees of most online brokers.​​​