gistfile1.go

type TxConfig struct {
	selector coinset.CoinSelector
	eligibleOutputs []txstore.Credit
}

func createTx(config TxConfig, outputs map[string]btcutil.Amount, changeAddr btcutil.Address) *MsgTx {
	msgtx := btcwire.NewMsgTx()
	addOutputs(msgtx, outputs)

	// Calculate the total amount needed to fulfil all outputs.
	amount := sumAmounts(outputs)

	// This one is pluggable; we just need a custom CoinSelector for VPs
	chosen := config.selector.CoinSelect(amount, coins)
	inputAmount := 0
	for _, coin := chosen {
		inputAmount += coin.amount
		tx.AddTxIn(btcwire.NewTxIn(coin, nil))
	}

	// Estimate fee. We may be able to use the standard one for VPs
	fee := estimateFee(...)

	// 
	change := inputAmount - amount - fee
	if change > 0 {
		addOutputs(msgtx, map[string]btcutil.Amount{changeAddr: change})
	}

	return msgtx
}

func (w *Wallet) txToPairs(pairs map[string]btcutil.Amount, minconf int) (*CreatedTx, error) {
	bs, err := w.chainSvr.BlockStamp()
	if err != nil {
		return nil, err
	}

	eligible, err := w.findEligibleOuptuts(minconf, bs)
	if err != nil {
		return nil, err
	}

	cfg := TxConfig{selector: MinNumberCoinSelector, eligibleOutputs: eligible}
	changeAddr := w.KeyStore.ChangeAddress(bs)
	msgtx := createTx(cfg, pairs, changeAddr)
	...
}

We still need to add the loop over the which inputs to include and calculating the minimum fee. Looking at the btcwallet code it seems they are doing this:
// 1. start with fee = 0
// 2. construct new tx based on the utxos
// 3. calculate minFee := minFee(tx)
// 3. if minFee < fee then done otherwise set fee := minFee and goto 1.