Most beginners trying their hand on lo-hi key photography falls to believes that over expose will delivers hi-key and under exposure will be lo-key, of which the truth is not like that.
On the contrary the exposure has to be perfectly accurate in order to get the best result of hi-lo key. This going to be a lengthy one ....for your sake.
Let's start from the beginning. Normal photograph will usually consist a variety of tones. Basically will be divided into hi-light ( white area ), midtone ( grey zone )and shadows ( dark/black area ) these are reflected in the eye droppers of levels and also histogram of photoshop image editing software.
When producing hi-lo key pictures, the effect/purpose is to eliminate those midtone grey zone and compress the subject and backgrd into a dual tone image. ( but there can be also some or little grey tone)
One very important key factor for these type of picture to excel is..high contrast.
Next...the subject has got to have a strong profile or outline contour so that the delineation will be neat between tone change.
Most successful hi-lo key pictures are done in a " controlled " situation.
" Controlled " meaning the photographer can contol the background, e.g. by providing a white backdrop, a window that create glare, a dark wall or black velvet curtain etc.
The subject also be " controlled " to dress in white or black to suit the effect going for.
Then the light source also got to be " controlled ".
Either an array of flood light shining thro a scrim to get those wash away hi-light effect or a focusing spot light just to pin-point on the subject and let the rest merge into the dark background.
A studio would be ideal but a corner of the room with some innovation also can achieve similar result.
Metering will usually be a spot meter or using the incident meter reading method instead of the reflected type.
Common hi-lo key subject/topic are usually portraiture, wedding, pictorial still life, commercial product, fashions etc.
Rarely seen topic using hi-lo key are...scenery, landscape, nature, close up, animals, foods illustration, documentary, journalist reportage, sports event, graduation, convocation, head of states group pictures etc.
If my research is correct ( stand to be corrected, anyone ) the Chinese black n white brush painting were the pioneer in using hi-key technique well before photography.
When painting koi fishes in a lotus pond, a few dab of black ink, carving out the lively profile of the fishes swimming and a leaf or two of the lotus and leaving the rest of the paper white/blank. Viewers will automatically interpret the white area as water. Isn't it amazing.
There are some photographers using IR black n white images to create successful hi-key landscape/scenery.
With these guide, hopefully, your next attempt will be improved.