We always recommend a heat press. Reason being is you can control the temperate and pressure to give you the same results every time.
That being said, you can use an iron. Set your iron to a temperature of about 140 degrees with no steam option.
Make sure you cover your transfer with another piece of fabric, don’t put the iron directly onto the vinyl as it may burn or damage the vinyl.
When pressing, make sure you can get a firm amount of pressure down on the transfer. Try to avoid moving the iron and keep it still. If the transfer is larger than the iron, do one part at a time. Lift the iron off the transfer, move to the next spot and press firmly. Make sure you remember what parts you’ve done so you don’t miss any.