Hi and thank you for allowing me to participate in this forum.
This may be slightly off topic. If the moderator removes or deletes it I understand.
As a professional leather smith, I have developed many tricks that my fashion school education didn't teach me. While the other posts are mentioning sewing machine maintenance I find that the most obvious problem for a beginner would be insuring that the two layers being sewn stay uniform with their lengths relating to the stitched seam. A typical flat bed sewing machine will inevitably drag the bottom ply causing it to stretch and inversely tug the upper ply causing it to gather, resulting in a misalignment in the joint, (i.e. the top gets shorter and the bottom gets longer). The solutions seem simple yet misinterpreted by the beginner. In traditional sewing a seamstress will pin the work pieces together to ensure alignment. While simple, not a very good practice when sewing leather. I have found evidence of 1/4 " ribbon being glued in to the seams typically in Chinese import pants. This practice seems like an expensive solution to a simple production problem, impractical in an American work environment. So I took the same basic idea and simplified it to the point of almost being ridiculous. I take a piece of plain old masking tape, cutting it into 1/4" strips and sticking it along the seam allowance. The tape itself is not stretchy by nature negating the stretchiness to light/soft leathers. The key is to remove the tape and eliminate any evidence of its presence. The unfortunate byproduct of taping is that it tends to leave some residue behind. Dealing with that residue, again, is another problem that needs to be solved. Some solutions could be to ignore the residue and let it disappear within the lining of a garment, or neutralizing the adhesive with something like talcum powder. Depending on the job, you may not want to introduce talcum powder. Used sparingly, it can do a good job but an unanticipated spill could potentially ruin your day. Another solution would be a chemical solvent like barge cement thinner or spot remover/dry cleaning fluid. Both of these options used in moderation will yield satisfactory results, but an error could be a catastrophe. Please note that this is only one of many solutions. Safety first should be a primary concern. When cutting masking tape into 1/4" strips I use a rotary knife and a straight edge. When applying solvents less is more. Small quantities of exposure will risk less damage to your work piece and your health. I hope you find this helpful.