Totally simple thing but I've never been able to do it because my pelvis refuses to tip forwards. People who I see doing it are all women or men who do karate or other flexible sports. Sure I can touch my toes but only with a severely rounded back.
To start working towards this, I like this sitting on a block idea:
When you sit down to practice your forward bend, check out the curve of your lower back and pelvis. Your pelvis should be upright or tilting forward, never backwards. Your lower back should have an inward curve. Use your ﬁngers to feel the shape of your low back if it is not apparent to you visually. If you feel your backbones protruding outwards towards your ﬁnger tips then you need to adjust before bending forward. Begin by turning your thighs in and widening your sit bones. If that does not adjust your low back enough, then prop your hips up by rolling the end of your mat up, sitting on a block, or sitting on a towel. Do not bend forward until your pelvis and low back are safe.
sensational-yoga-poses.com advises doing this too:
If your hamstrings are really tight (like iron or steal that will not bend, even if heated to super high temperature), you'll be reaching upwards more than forwards each time you inhale. If this is the case or hamstrings are so tight that you can't even sit upright with one or both legs straight, then sit on blocks or edge of a sofa or chair (or table) so that you upper body does tilt forwards.
Then as you inhale focus on lengthening your upper body away from your pelvis and as you exhale allow your ribcage and head to sink down.
By practicing daily you may find that you can gradually reduce the height of your prop. (Don't go sawing the legs off of your sofa. Just find something lower to sit on.) As you do reduce the height of your prop, remember the feeling of keeping your spine long and also the feeling of tilting your pelvis forwards.