There are 2 great ways to grow a hedge. You can simply line up a series of plants in a row or you can used the double/triple staggered row layout. The choice depends on the hedge plant and its growth rate, the purpose of the hedge, the environmental factors and the space you have to work with. The space you have to work with is probably the most defining factor as a small space will require the single row method.
In a single row format you lay a string line and measure your holes depending on the plant, so for example a Photinia would be around 60cm apart in a well ventilated full sun position.. In a staggered format you place plants in successive rows using alternating distances on those rows. Depending on the space you have you may be able to get more than just a double row.
Let's say that you have a slim garden bed along a fence line and you want to block out neighbours, then a single row of Thin Red™ Photinia x fraseri would be great. First, check the soil is correct for the plant, avoiding high alkalinity and correcting poorly draining soils with organic material and/or gypsum. Measure out your holes, dig twice as large as the root mass and fill with water. Fill the bottom of the hole with a small mound of the soil and compost mix. Using a bucket of water with a dilute liquid fertiliser, dip the root mass in and try to tease out roots a little so that when planting you can spread those roots across the mound. Fill in with more soil and compost mix up to the root collar and build a raised wall around to help retain water. Lay a slow release fertiliser and a monthly spray of liquid fertiliser is going to help.
Lay your irrigation and for a plant like the Photinia x fraseri (that doesn't like lot of moisture due to fungal diseases) be careful of anything generating a lot of spray. They are very drought tolerant once established so keep the water up initially, but look for signs like the ends of leaves browning in cooler months as this can be a sign of overwatering.
As another example let's say you wanted a dense barrier that aided in sound reduction and you had a bit of space to do it. Try Juniperas chinensis "Spartan", which grows in a dense column. Dig a double or triple trench keeping the parent soil aside to mix with compost. Again, measure out your holes, dig twice as large as the root mass and fill with water. Fill the bottom of the hole with a small mound of the soil and compost mix and after teasing out roots spread across the mound. When fully planted, lay slow release fertiliser along the trench and return the soil and compost mix. This will provide a nutrient rich and loose soil for the roots to search and grow through.
Pruning requirements are different for each plant but as a general rule it's not a bad idea to pinch out a few early spring shoots to encourage more dense growth. Just make sure don't chop too hard in summer and burn back a section like this bloke in the drawing above.
Keep a bottle of eco organic fungicide on hand and watch for powdery white build ups particularly on Photinias and Lily Pilys. Look regularly for pests, check under leaves for eggs. Look on trunks for scale.
Consider whether staking is required, particularly with single row hedges with little shelter.