instant
Describes foods or products that are instantly soluble or ready to use, usually by simply adding water or another liquid.
Wenn es schnell gehen muss, trinke ich einen Instantkaffee.
When things have to be quick, I drink instant coffee.

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Describes foods or products that are instantly soluble or ready to use, usually by simply adding water or another liquid.
Wenn es schnell gehen muss, trinke ich einen Instantkaffee.
When things have to be quick, I drink instant coffee.

'Instant' is a loanword from English and is almost always used in German as the first part of a compound noun (e.g., 'Instant-Kaffee' (instant coffee)). In this function, it is not declined and is often written with a hyphen.
Für die Reise habe ich mir eine Packung Instant-Nudeln gekauft.
For the trip, I bought myself a pack of instant noodles.
In the food sector, 'instant' refers to products that can be prepared quickly with liquid. Common examples are 'Instantkaffee' (instant coffee), 'Instantsuppe' (instant soup), 'Instantbrühe' (instant broth), or 'Instantpüree' (instant mashed potatoes).
Morgens trinke ich nur einen schnellen Instantkaffee.
In the morning, I only drink a quick instant coffee.
The meaning 'sofortig' (immediate) is used especially in the digital and technical field. Typical terms are 'Instant-Messaging' (as with WhatsApp), 'Instant-Zugriff' (instant access), or 'Instant-Feedback' (instant feedback).
Die App bietet Instant-Messaging mit allen Kontakten.
The app offers instant messaging with all contacts.
direct
'direkt' and 'instant' can both have to do with something immediate. 'direkt' is a normal German word and, in the time-related sense, means: without delay. 'instant' is used much more narrowly in German. With foods, it means that a product is ready quickly, for example with hot water. In technical or digital contexts, 'instant' can also mean 'immediate', but it sounds clearly more English. The risk of confusion comes from the fact that 'instant' often means 'sofort' in English. In normal everyday German, 'direkt' is usually more natural; 'instant' fits mainly with products or certain digital terms.

Für den Campingausflug nehmen wir Instantkaffee mit.
For the camping trip, we are taking instant coffee with us.
Nach der Reise gehe ich direkt ins Bett.
After the trip, I go straight to bed.