is a combination of the 副助詞（ふくじょし） (adverbial particle) だけ
, and the 格助詞（かくじょし） (case-marking particle) で
. This expression is regularly thought of as meaning ‘just by (A), (B)’. However, the literal translation is a bit closer to ‘with only (A), (B)’. This reflects the で
particle’s standard usage of showing a ‘tool’ that is being used to achieve a certain result.
To use だけで
, simply attach it to the end of any (non ます) form of verb, or noun.
Just talking with my girlfriend is fun. (We don’t need to do anything else, because talking is fun enough)
This is just a microwave, an oven function is not included.
だけ differs from しか in one very important way. だけ highlights that there is/was more than one option, but that only one thing was chosen/used. However, しか implies that there is only one possible option to begin with.