If your wall already painted then no need apply sealant also can

The step would be fixing imperfection on wall, filling up holes and smoothen the wall by sanding or grinding if you want

Then use acrylic emulsion paint as base coat then 2 layer of finishing coat or 1 layer of the colour of similar to base coat

Depends on the colour of your current wall, if it is dark colour and you want to paint to light colour you might need 2 or more layer of base coat to cover the colour so that your finishing coat don't look weird

If you are just repainting with the same colour you can directly go for finishing coat too