Hold the first bracket against the wall under the windowsill.,
Drill the holes over the marks.,
Insert a wall plug.
Hold the bracket over the drilled holes, in position.,
Place the shelf board on the bracket and hold it across to the other side.,
Add the second bracket as for the first bracket.
Attach the shelf board to the brackets.,
Polish or varnish the wood if desired.
Mark its drilling holes with a pencil. The bracket should be flush under the windowsill so that the shelf board will sit out evenly from the windowsill. Check that this will be the case by holding the shelf board and bracket in place. Make adjustments as required.
If the shelf board isn’t high enough, you’ll need to either find a new shelf board or add an additional board layer (glue it to the first board). If there is only a slight dip between the sill and the shelf board, this is probably going to be fine and your cat won’t be bothered.
Check that no matter how the shelf board is placed, you can still open and shut the window.
, Drill far enough to allow the screws to penetrate into the wall firmly.
Be sure to check for electrical wiring and plumbing before drilling. Being under a window, it’s probably going to be okay but always check to be safe.
Use a masonry drill bit.
,, Attach the screws and screw them in firmly.
, Place a spirit level on top to check it is sitting level. Make marks on the wall for the second bracket.
,, Screw the board to the brackets from the underside.
, The shelf is now ready for kitty to laze about on, watching the world pass by.