Well much of what is needed can be found directly under another similar article on Helium written by me of course, and that is How to Make Slipcovers for your sofa & sofa cusions. http://www.helium.com/items/1152220-how-to-make-slipcovers
The article focuses mostly on making a slipcover for a sofa though the steps are very similar if not identical right down from measuring, choosing your fabric and finally sewing the covers. It's a great read, though I will list a few key things needed in the article below.
Slipcovers are not hard to make at all and give any space weather it be a sofa, pillows, your favourite lounge chair or even a quick folding chair a lift and a boost of colour. The very first thing to take into mind is the type of fabric you want to use. The choice depends of how often this chair is used and the durability of the fabric according to the weight and texture of it. Begin by measuring each side of the chair such as the entire length of back, the seat and front length from the edge of the seat, and also the side panels from the seat all the way to the ground.
The next step is to take the measurements with you to your local textile store to search for your fabric. Please note that many awesome and great quality fabrics can be purchased at your local thrift stores. Just ensure there is enough fabric you choose and buy keeping the durability and quality in mind. Look for little heavier fabrics as they will not only be easier to work with but are more durable especially for a chair. Stay away from light silks and satins as they slip and are super hard to work with for this type of pinning and sewing, not to mention not that durable for a seat and easily wears and tears.
One of the important thing to remember is to use a colour matched thread that also goes according to the weight of the fabric of your choice. Nylon and polyester threads are strong and better due to the fact they do not shrink. This leads me to another very important suggestion. It is necessary to wash and dry your fabric prior to any cutting and pinning. It is to avoid shrinkage, as it would be a huge waste of time to sew a slipcover only to have it shrink in your first wash. I am speaking from personal experience here so do take my word.
Now with all these few important things out of the way, we may begin. Unfold the chair you are working with. Drape over the fabric with the raw face upwards and the finished side downwards non visible to you. Ensure the back is touching the ground and front also has plenty of fabric on the floor. Now depends on the type of chair you are working with, I will assume just for simplicity of writing this it's a simple folding chair without any ornate designs.
With smooth stroke ensure the fabric is covering the seating area entirely, keeping an eye on the back making sure it's still touching the ground with at least an inch of fabric to spear for finishing edges. Begin pinning with pins the edges of the fabric close to the chair, sort of outlining it with your pins around the length of the chair up to the seat crease on both sides. (Do not pin the fabric too tight together as it still needs to be reversed when completed and will not fit when flipped.) Then as you have done so, from the additional fabric you have cut the size of the side panel of the chair below the seat from the back to the front leg, leaving few extra inches on each side of the entire square for pinning. Pin the section along with the edges of the seat side, back leg side and front leg side to the square.
Now if you made a template or took measurements, cut another identical piece for the other side of the chair. The reason not to cut two at a time is in case of inexperience to avoid a mistake and wasting two pieces of fabric rather then just one.
So, your entire chair is now pinned, with all the raw fabric sides outward and do double check that the side panels are raw side out. It is a mistake easily made. Slowly removing the entire piece, begins sewing along the pins and pulling them out one by one as nearing to them. Sew all over the areas that were pinned, and do not worry about excess fabric as this will be cut off. Once you have sewn your entire cover, cut the excess fabric leaving about 4 inches on each side for now. Flip the entire slipcover the right side out and do a fitting over your chair. If you are satisfied with the fit and look, this is the time you can cut off little more of the excess fabric on the inside.
Once your cover is nearly completed, drape it over your chair and tuck in the length also pinning it evenly all around the chair. This will create a nice clean edge. Sew over the pinned area to finalize your cover. You are now done. Now was that hard? Enjoy your new look and get creative with tying a tassel around the back of the seat or stencil something personal on the backrest of your chair cover.