Places to Avoid
Many of the tailors you are likely to find strolling around the Orchard Road or City Hall areas are best avoided. These shops pay high rent at the front or ground level of shopping centres, and tend to employ one or more touts to man their doors soliciting for customers, offering gratuitous quotes, etc. Their shops will often have an attractive display of fabrics, and manikins with display pieces of clothing.
What you won't see is a pair of scissors, or any pieces of clothing in progress. These shops employ "tailors" who simply measure customers and record orders, to be made off-site (often in neighbouring Indonesia or Malaysia) by a cutter who has never met the customer. The tailor you meet in the shop is really just a salesperson who happens to be selling made-to-measure clothes, but has no experience designing custom patterns or cutting.
Since the clothes are made offsite at a cheaper location (and because these tailors target tourists on short visits) it is impractical to transport them back for a fitting on the customer before they are finished. As there is no opportunity to make size adjustments, the cut will err on the side of being too big, most noticeably at the waist of trousers, and armholes of suit coats.
The end result is clothes that often fit no better than a carefully chosen ready-made piece from a shop, but not as well constructed. There is simply no point in having clothes made this way, as it doesn't give the perfect fit that you expect from tailored clothing.
As a general rule, any shop that has a street fronting, is on the ground floor, or solicits business from passers-by can should be completely avoided.
Working With Your Tailor
Although a good tailor is a skilled craftsman who can produce amazing clothes, it doesn't always happen effortlessly. Your tailor will have made thousands of pieces for other people, but that experience doesn't mean they'll instantly know what you want.
Some tailors will ask for incredibly detailed instructions on the minutiae of your new garment (right down to the size of belt loops for trousers and whether pockets should be buttoned or flapped), taking nothing for granted. Others will tend to only ask about a few major style choices, sticking with convention for the rest unless you make a specific request.
If there is a specific style you want, do make a point of mentioning it and making sure it is written down. Your tailor should be able to entertain the request, or possibly explain why it isn't a good idea for your particular cut, shape, fabric, etc.
Ideally, you can start of with perhaps one shirt or pair of trousers to start a relationship with your tailor, which you can then wear for at least a day to try the fit, before ordering more pieces. This way, you can try the tailor's work and style without a major outlay, and make any changes you'd like before getting more work done. A good tailor will understand and be happy to entertain your initial small order in the hope of building a longer partnership, and it's worth being wary of any pressure to place a larger order at this initial stage, as that would tend to suggest the tailor doesn't get a lot of repeat business!
Shirts are nearly always made and completed offsite, which will take around a week.
Trousers and Suits
Trousers and suits should be cut by the tailor himself, at or near his shop so that you can come back for a fitting. Although a good tailor will be understanding of a customer's tight schedule and urgent jobs, the more time you can allow the better. Ideally, you should allow a week for this work to give your tailor plenty or time, although they might be able to complete a piece of a few days if asked. If you need your work completed quickly, it pays to be flexible in your timing and offer to come back for your fitting late in the day if required.