Decide how far apart the two pieces are
A very formal blazer beside heavily distressed jeans creates a sharp contrast. A soft jacket with washed denim makes a quieter combination. Before adding shoes or jewellery, decide whether that contrast is what you want. There is no rule that the two pieces must match in mood, but knowing which difference you are choosing makes the rest easier.
Look at the jacket's fabric, shoulders and buttons. A smooth dark blazer with strong shoulders can dominate a pale pair of jeans. A textured jacket with a relaxed front may blend more easily. If the jacket feels too important for the outfit, try a plain T-shirt or an open-neck shirt underneath before buying a different one.
Keep the middle comfortable
The blazer should close if you intend to wear it closed. Even when worn open, it needs enough room across the upper back to reach for a bag or sit at a table. Jeans with a high waistband can meet a shorter top neatly beneath it; lower-rise denim may call for a longer layer if you want coverage when moving.
Check what happens around the pockets. A thick jumper, a tucked shirt and a rigid waistband can create several layers in the same small space. Remove one source of bulk rather than repeatedly pulling the jacket straight. A blazer does not have to be tight to look considered.
Connect the outfit with a shoe
A loafer brings some of the jacket's structure down to the ground. A plain trainer can bring its formality down instead. Boots may work with either, depending on the shape and finish. Choose the shoe for your route and the time spent standing, then check the trouser hem against it.
A belt is optional. Add one when you like the line it creates or need it to hold the jeans in place. If the waistband already fits and the jacket covers it, another visible buckle may add little. The same applies to a large bag or several necklaces: every part does not need to make a statement.
Try it with the jacket off
An outfit for lunch or work will probably spend part of the day without its outer layer. Make sure the shirt or top underneath is something you are happy to wear on its own. That small check often makes a blazer-and-jeans combination feel less arranged and more like clothes you can actually use.
