▲你的句子完全正确，方式副词有时是可以放在句首的（当然也可以放在句末），所以你的两个句子都是对的：

Slowly depress the accelerator / brake pedal. 慢慢踩下油门／刹车踏板。

Depress the accelerator / brake pedal slowly. 慢慢踩下油门／刹车踏板。

▲更多例句：

Slowly bend from the waist and bring your head down to your knees. 慢慢弯下腰，把头低垂到膝部。

Slowly add the flour, stirring until completely blended. 慢慢加入面粉，搅拌至完全混合。

▲再举一个 carefully 的例子：

Carefully peel away the lining paper. 小心剥掉衬着的那层纸。

Carefully space the words on the poster. 仔细地把海报上的字隔开排好。

Carefully centre the photograph on the page and stick it in place. 把照片小心地放在页面中央并粘贴好。

Carefully tighten the clamp until it firmly supports the pipette in a vertical position. 小心翼翼地把夹子夹紧，直到能把移液管牢牢地垂直固定好为止。