The following is a fairly comprehensive guide in how to remove the gears on your bike and change them to a single speed. This guide will talk about removal of the cassette. Adding your single gear, chain tensioner and chain as well as removal of the chainset and adding a single chainring. As a difficulty rating I would give this a 4 out of 5. The rating is high more because there is lots of things to cover here but each individual step isn't that difficult to do.
You will need Specialist bike tools for this job and there may be variations depending on your drivetrain. To start you will need to buy the conversion kit. I've used a 32t front here on a 16t rear, having a 2:1 ratio which is good place to start.
Allen key Multi Tool
Cassette Removal Tool
Crankset removal Tool For either Hollow tech or splined cranks
Workstand (Optional but it helps!)
Get your bike set up in your workstand or somewhere that supports your bike in a solid position.
Remove the rear wheel from the frame. The tools you'll need for this job are the Cassette Tool, An Adjustable Spanner if you don't have an all in one cassette tool, and Chain Whip.
Using picture above as guide put you cassette tool into the lock ring. I reattach the quick release over the tool to stop it slipping off when twisting. Using the chain whip to hold the cassette in place turn the tool anti-clockwise to remove. the Cassette should just slide off from the wheel leaving the bare cassette body. Sometimes the cassette may not slide off easily in which case use a small rubber mallet to tap it loose.
Clean the cassette body removing any grime or grease. Next, it's time to get your spacers cog and lock ring. These slide easily onto the cassette body but now comes the hardest part of the exercise, knowing what spacers to place in which order. It is a bit of case of trial and error. Your aim will be to have a nice straight chain line from your back wheel to the chainset meaning less rub on the gears and a smoother ride. More likely the cog will need to be closer to the outside of the cassette body.
With all the parts put on, screw back on the lockring holding the parts in place. Use just the cassette tool to tighten it up (no need for chain whip).
Your finished wheel should look like the picture above. Remember you may have to repeat this process a few times to get the chain line right in later stages.
Using your Allen keys undo the bolt to remove the cable to the rear derailleur as shown by the left hand arrow. The right hand arrow shows the bolt that will be need un-screwing to remove the rear derailleur from the frame.
At this point the chain will need removing. If you have a Sram chain the you can unlink the chain by pushing the chain together where the gold power link is. If not then you will require a chain tool. Push the pin nearly all the way out (leaving a little in case you want to reattach) with the tool to split it. Remove the chain from the derailleur and Chainset.
Once the derailleur and the chain have been removed it's time to attach the Chain Tensioner. This is put in the same place as where the rear derailleur was bolted in. Lightly grease the threads before attaching
When attaching the Tensioner pay attention to the small pin at the back. In the picture above (taken from behind the frame hanger) you can see the pin sits up against the frame hanger. If it is put on the wrong side, the tensioner will slip and not work.