Answer: don't! Just attach cheap IoT devices to the machines directly and leave SCADA to do its thing (which it's very good at).
The control and automation of industrial processes, such as manufacturing and energy grids, is usually implemented by a SCADA system - mature technology which is good at providing high-reliability control and monitoring of safety-critical, real-time processes.
SCADA typically has embedded controllers attached to individual machines, running basic "ladder logic" programs, which can then be networked across a factory to higher level systems for production control. SCADA is a technique, not a standard, so each system contains many different standards and often proprietary technologies too. Historically, SCADA systems haven't been connected to the internet, partly because of over concerns over security and partly because the protocols are not IP-compliant (and there's no "standard" way of talking to a SCADA system over the internet). Thus, each SCADA system is a closed, monolithic, unique entity.
Modern companies increasingly rely on the internet, and specifically the web, to run their businesses, often using a suite of SaaS applications running in the cloud, where they can integrate all the information they need to run and optimise their business - increasingly important in today's "just in time" world. The Internet of Things is a natural extension of this approach out into the real world - if you deploy a modern sensor or controller then of course you're going to want to integrate it with all your other business processes. Viewed from this perspective, legacy SCADA implementations can appear to be outdated, unconnected obstacles to business modernisation.
So how to square the circle?
Firstly, let's address concerns over security, which are often misplaced: industrial systems have often relied on "security through obscurity" which - as Stuxnet demonstrated - is little defence against a determined attacker. Today's cloud systems by contrast - being by their very nature open to attacks in all forms - have had to develop excellent best-practice techniques to keep themselves secure.
We've seen a number of our customers successfully overcome the challenge of opening up existing SCADA-controlled systems to integrate them with modern business systems. But they've done it not by putting the SCADA system itself online, but by leaving the SCADA system well alone (it's doing a good job, so why mess with it?) and simply retrofitting new IoT sensors on top (for example, the lovely new sealed-for-life sensor tiles from Disruptive Technologies).
For example, to achieve the important goal of optimizing the energy use in buildings or factories, great results have been obtained by retrofitting low-cost IoT sensors to measure the temperature of pipes and rooms and the consumption of energy, and thereby gain all the insight needed to understand how to optimize energy use. The "closing the loop" of the optimization might then be implented either by tweaking the SCADA system manually, and/or possibly by adding a simple IoT-controlled switch to make crude interventions such as shutting off a machine when it isn't needed.
This selective retrofit approach also allays some security concerns - sure, there's now a new potential path for information leakage, and potentially even for hackers to control some things, but it's very selective - this new connection cannot be used to, e.g. compromise critical networks, reprogram assembly line robots, or otherwise interfere in processes which are business- or safety-critical.
Meanwhile, as internet technologies like Ethernet slowly gain a foothold within SCADA systems, in time we'll see SCADA systems opening up to take their place as first class citizens on the Internet of Things. But this could take another decade or two to complete, and meanwhile the retrofit approach can quickly pay dividends.