My knitting pattern says for the right front side to reverse all shaping. On the left side I had to cast off 3 stitches at the beginning of a row, and then decrease at the beginning of alternate rows x 5.
Does this mean that I cast off at the end of a row? If so how on earth do you do that??? Totally perplexed by knitting jargon 
Can you provide a link to the pattern you are doing? This will help us to understand what you are working on 
What can be done decreasing will also work the other direction. One of the benefits of knitting forwards and backward is you can then see what is on the front of the work at all times. It also tends to make on ambidextrous. Decreases which are typical are Knit 2 Together (K2Tog) which are right leaning or Slip, Slip, Knit (SSK) and these are left leaning. Your knitting style also helps determine what decreases you would want to be using.
There are some useful tutorials on YouTube for doing decreases and this may help you. But tutorials should be worked on scrap yarn big enough to make a swatch and then tested before you get back to your knitting.
It is a learning experience. With casting off, two methods, is you take the stitch to knit 2 stitches, passing the first one over the last one knit on the right needle, repeating to the number of cast off, or doing a series of Knit 2 togethers, each time passing the stitch you finished back to the left needle and knitting through the back of the stitch. The second method is stretchier and may make a difference in how well a garment will fit. If you look at the other end of the work, what can be passed over left to right is also able to be passed the opposite direction at the opposite end of the needle. Only then it will be tight if you are not also knitting to supply yarn to these. Stitches worked with yarn are working live stitches. If you pass stitches without live yarn being there, they will be tight. This added tension can be felt when wearing a garment. You are working with live stitches such as at the beginning of a row is usual method of mirroring sides. One row difference is not going to make it fit querky or look odd.
Most people who write down patterns did not go to the school of how to right patterns (if there is such a thing) or consider that the person knitting may not be an expert or even be knitting in a different knitting style.
When you are working with pattern stitches, decreases and cast offs need to be worked right for the sake of fit. The tendency of new knitters is to kill the thing and knit so incredibly tight, that this does not look right. That is why I swatch, so I am confident when I do something and don’t make a mess of my yarn.
A good way to learn the names of your decreases is to say them out loud, it helps with memory. Some people call them by right leaning or left leaning. So to learn the jargon I say them out loud or under my breath if others can hear.
Hope you can solve your problem and work it through.
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It’s not practical to bind off at the end of a row, so work to the end of the row, turn and bind off at the beginning of the next row. You can then decrease at the beginning of the alternate rows 5 times to match the left side.
I said as much but told why it isn’t done, until people know why they don’t do it, they will find it is too tight. We learn from our own mistakes, from proving what is reasonable to ourselves. People who do so learn to think and will be able to solve their own problems. Which is the idea or teach a man to fish vs. hand it to him.
Thank you all for your replies - I realised I didn’t have to cast off at the end, just knit from the other side.
I’ve got all 5 pieces now - just have to join them together, but I’ll try to figure it out first.