Patching and Plastering - drywall and knockdown questions

i am building a wall partition and i have a couple questions. in the inside corner where the new drywall meets the existing wall (which has a knockdown texture), and in the corner where the new drywall meets the ceiling (which also has a knockdown texture), what is the strongest way to tape both of those joints and then apply knockdown texture to the new drywall, blending it in with existing wall and ceiling texture? i also have an area with butted seams, one side which is new drywall and the other a knockdown textured wall. do i need to sand down the texture a few inches in, tape the corners and seams, and blend the texture? is this a job for a professional, or is it something that a pretty handy person could do? also, i've never done taping and texturing before.
thanks a lot.

Welcome to the forums

A lot depends on how heavy the knockdown texture is. If its real light - just tape over it. Heavier textures should be scraped/sanded down some before taping. The butt joints will be the hardest to blend in, the more texture you remove the easier it will finish.

Knockdown is one of the harder textures to blend in but it isn't rocket science.