Two kinds of liquid. Acrylic liquids, or monomers, mix with acrylic powder to make the bead you sculpt with; most listed here are labelled EMA, including Young Nails in 6 and 32 oz, Mia Secret in 8, 16 and 32 oz, and LAVIS by the gallon. Dip liquids are the steps of a dip system, such as SNS Gelous Base, Sealer Dry and Gel Top, or the NuGenesis Base Gel, Activator and Finish Gel.
For dip, use the liquid system made by the same brand as your powder and follow its labelled order of steps, because the steps are not interchangeable between brands. For acrylic, each brand advises its own liquid for the best result, and several say their monomer works with other powders, so test a few nails before you switch a whole service.
Prep the nail first: push back the cuticle, shape, remove the shine, cleanse and dehydrate. Then apply a thin coat of primer to the natural nail only, keep it off the skin and let it air-dry before the first bead. This page lists acid-free prep such as LAVIS pH Bond and the LAVIS acid-free dehydrator, plus OPI Bond Aid.
A runny bead usually means too much liquid in the brush, and a dry, grainy bead means too little. Wipe the brush on the edge of the dappen dish to control the liquid, and pick up powder until the bead looks glossy and holds its shape on the nail. Each brand prints its own guidance on the label, so follow it for the system you use.
Keep bottles tightly closed and upright, away from heat and direct sunlight, in a cool, dark place. Work in a well-ventilated space and pour only what you need into the dappen dish. Cap dip liquid bottles between clients and wipe the bottle neck before closing so the cap does not stick.