Prep

Before you start doing the walls in a room try to remove as much from the room as possible.

Stack everything that's left in the middle of the room, leaving enough space to move a stepladder or stool around next to the walls as you will need room to work.

Lay plastic (visqueen) or dropcloths on carpet or wood floor to protect from any drips or paint splatter.

Tape off base board and around sockets and switches to avoid any overbrushing or roll splatter on woodwork or switches.

Product selection

Interior walls are normally painted with latex paint, interior paints come in the following finishes: satin, egg shell, semi gloss and flat.
Satin and flat finishes are better to use if the walls are not too smooth as (shiny) gloss and eggshell finishes show every small imperfection.

The process

Cut in along the ceiling line, the corners, round the architrave, base board and around sockets and switches before using the roller but only one wall at a time to avoid what the pro's call (flashing). A difference in sheen to to partial drying of cut before rolling.

Start in a top right-hand corner(if you are right-handed and vice-versa) and work away from the main source of light(e.g. window).

When using a roller, Start in a top right-hand corner(if you are right-handed and vice-versa) roll no more than 2 roller widths per dip flip the roller and lay off (roll back over)the paint as you go.

Roll about a 3 foot width at a time. If you want a solid coat on the walls /ceilings then use the 'backroll' method. This is handy if you want to try and cover in one coat.

To Backroll: Roll about a 3 foot width at a time using plenty of paint. When you have rolled the third foot then go back and re-roll the first foot without putting more paint on the roller. This is very effective especially on ceilings.