Anyone who has tried making or modifying clothes without prior knowledge realises that it's not that easy, and takes a bit of trial and error. Some years ago I used to try modifying jeans and other pants to make them tighter in the right places. It didn't take me long to decide that with my level of skill, it was a better idea to pay for alterations or else to buy another pair.
These days my sewing is mostly confined to fixing small tears in such places as the underarm of shirts, places where they tend to tear and the tear is easy to fix. During my backpacking travels I found a sewing kit was essential. Not only was it a lot cheaper to fix torn clothing than to replace it, in may places replacement was not an option, becuase my size simply was not available.
On one occasion I eventually decided that a pair of jeans was no longer wearable - but I kept them to use the denim as patch material. I am still quite proud of the moneybelt that I made by taking a rectangle of denim, folding it and sewing it, and attaching the zip from the jeans - then completed the job by threading a long shoelace through it to tie around my waist. It was far more practical and comfortable than some of the synthetic material moneybelts I have owned.