It is relatively easy to make your soil more acidic and there is certainly more than one way to go about it. However, you should avoid abruptly raising your soil's pH with quick-acting agents. You might be excited to get those acid-loving plants into the ground, but an abrupt pH change will likely kill the soil. Then you won't be able to plant the plants you had in mind--or anything else--until the soil recovers. To avoid soil shock, stick to natural soil amendments and only raise your soil one pH point per growing season.
Test your soil's pH. You can do this yourself with a pH meter, or you can take a soil sample to your local extension office where staff will test it for you. Some home-improvement centers like Lowe's offer the service for free. If you take your sample in to be tested, be sure to call ahead for instructions on how to prepare and package the sample.
Use compost as a buffer agent. If your soil's pH is only one pH point or so off, adding compost will give it a buffer of one pH point (so, soil with a pH of 7 would be able to grow plants that require a pH level of 6 to 8). Simply spread a 4-inch layer over the soil and then turn it into the soil to a depth of 8 inches. Compost can also be used to buffer soil that must wait another growing season to be raised another pH point.
Add sulfur. A little sulfur goes a long way, which makes it great for large planting areas. And it is by far the safest and most effective method. However, it takes several months to change your soil's pH so it should be applied to the soil in the fall before a spring planting. To make your soil more acidic by one pH point, add .13 oz. of granular sulfur per square foot of sandy soil and .26 oz. per square foot of non-sandy soil according to the manufacturer's instructions.
Add peat moss. Spread a 2-inch layer of peat moss over the soil and work it into the topsoil to raise your soil's pH by one point. The soil can be planted immediately. But be aware that adding peat moss will also significantly increase your soil's drainage. If the plants that you have in mind need sand or clay-based soil, this method is not ideal.