Stupňování přídavných jmen

The comparative

Two ways, both correct:

• Simple, with -эйшы / -ейшы: новы → навейшы, стары → старэйшы, цёплы → цяплейшы. Preferred for short adjectives.
• Compound, with больш: больш цікавы, больш зручны. Preferred for longer ones.

Never both at once: больш навейшы is wrong.

The irregulars

Than

This is the point to get right. Belarusian uses за with the accusative, or чым with the nominative:

• Нёман даўжэйшы за Свіслач.
• Нёман даўжэйшы, чым Свіслач.

A bare genitive - the ordinary Russian pattern - is not used here, and it is the single most frequent error in the topic.

The superlative

• самы plus the adjective, and самы agrees too: самы вялікі, самая прыгожая, самае цёплае.
• the prefix най- on the comparative: найлепшы, найбольшы - shorter, and common in writing.
• за ўсё / за ўсіх after a comparative: лепш за ўсё - best of all.

Adverbs compare too

хутка → хутчэй, добра → лепш, дрэнна → горш. These are the forms you already used impersonally: Сёння мне лепш.