Research First, Construction Second
Historical costuming without research produces something that looks historically-adjacent but isn't historically informed — which is fine for fantasy or Renaissance Faire purposes but misses the point if your goal is recreation. Start with primary sources: surviving garments, period portraits and illustrations, and contemporary descriptions. Janet Arnold's Patterns of Fashion series is the gold standard reference for Western European garments from the 1500s–1900s.
Choosing a Period
Narrowing your period significantly reduces research complexity. Pick a specific decade or time period (not "medieval" — which spans 1000 years — but something like "1580s Elizabethan English court" or "1870s Victorian bustle era") and research that period specifically. The construction methods, materials, and silhouettes change dramatically across history, and trying to cover too much ground produces a confused result.
Accuracy vs Wearability
Pure historical accuracy is sometimes incompatible with all-day wearable costumes. Period undergarments are different from modern ones; period construction can be very time-consuming; period materials can be expensive or unavailable. Decide what your accuracy goal is before you start: museum recreation, historically-informed costume, or fantasy-influenced historical aesthetic. Each is legitimate and each requires different decisions.